diff --git a/crates/ui-egui/src/header.rs b/crates/ui-egui/src/header.rs
index f013e5b7..40472f52 100644
--- a/crates/ui-egui/src/header.rs
+++ b/crates/ui-egui/src/header.rs
@@ -164,6 +164,9 @@ pub fn show(app: &mut EffectcraftApp, ui: &mut egui::Ui, rect: Rect) {
     if app.ui.tool.puppet_kind().is_some() {
         x = puppet_options(app, ui, &p, x, cy);
     }
+    if app.ui.tool.roto_kind(false).is_some() {
+        x = roto_options(app, ui, &p, x, cy);
+    }
     let snap = Rect::from_min_size(pos2(x, cy - 10.0), vec2(20.0, 20.0));
     // The engine owns snapping (View ▸ Snapping); the checkbox mirrors it.
     app.ui.snapping = app.session.state.snapping;
@@ -481,6 +484,26 @@ fn paint_swatch(p: &egui::Painter, r: Rect, paint: &ToolPaint, hovered: bool, t:
     p.rect_stroke(r, 2.0, Stroke::new(1.0, if hovered { t.text } else { t.field_border }), StrokeKind::Inside);
 }
 
+/// Roto Brush / Refine Edge tool options: the brush Diameter (⌘/Ctrl-drag in the Layer panel
+/// sets it too, as in After Effects). Returns the next x.
+fn roto_options(app: &mut EffectcraftApp, ui: &mut egui::Ui, p: &egui::Painter, mut x: f32, cy: f32) -> f32 {
+    let t = app.tokens;
+    let refine = app.ui.tool.roto_kind(false).is_some_and(|k| k.starts_with("refine"));
+    let o = &app.session.state.roto;
+    let (key, v) = if refine { ("refineDiameter", o.refine_diameter) } else { ("diameter", o.diameter) };
+    let label = tr("Diameter:");
+    let lr = p.text(pos2(x, cy), Align2::LEFT_CENTER, label, Tokens::ui(12.0), t.text_dim);
+    x = lr.max.x + 6.0;
+    let (r, nv, _) = widgets::hot_number_at(ui, pos2(x, cy - 9.0), egui::Id::new(("roto-opt", key)), v, 0.5, (1.0, 5000.0), 0, " px", &t);
+    app.auto.add("header.roto.diameter", r, label);
+    if let Some(nv) = nv
+        && let Err(e) = app.session.execute("roto.options", json!({key: nv}))
+    {
+        app.ui.status = e.to_string();
+    }
+    r.max.x + 12.0
+}
+
 /// Puppet tool options: Mesh: Show, Expansion, Density (for new meshes and the selected
 /// layer's meshes). Returns the next x.
 fn puppet_options(app: &mut EffectcraftApp, ui: &mut egui::Ui, p: &egui::Painter, mut x: f32, cy: f32) -> f32 {
diff --git a/crates/ui-egui/src/i18n/ui.rs b/crates/ui-egui/src/i18n/ui.rs
index d1e81f1e..ce7c2a99 100644
--- a/crates/ui-egui/src/i18n/ui.rs
+++ b/crates/ui-egui/src/i18n/ui.rs
@@ -286,6 +286,7 @@ const JAPANESE: &[(&str, &str)] = &[
     ("Maximize Panel", "パネルを最大化"),
     ("Media Replacement", "メディアの置き換え"),
     ("Mesh:", "メッシュ:"),
+    ("Diameter:", "直径:"),
     ("Method Used: {}    Average Error: {} pixels", "使用した手法：{}    平均誤差：{}ピクセル"),
     ("Models folder: {}", "モデルフォルダー：{}"),
     ("Modifiers:", "修飾キー:"),
@@ -721,6 +722,7 @@ const SIMPLIFIED_CHINESE: &[(&str, &str)] = &[
     ("Maximize Panel", "最大化面板"),
     ("Media Replacement", "媒体替换"),
     ("Mesh:", "网格："),
+    ("Diameter:", "直径："),
     ("Method Used: {}    Average Error: {} pixels", "使用方法：{}    平均误差：{} 像素"),
     ("Models folder: {}", "模型文件夹：{}"),
     ("Modifiers:", "修饰键："),
@@ -1244,6 +1246,7 @@ const TRADITIONAL_CHINESE: &[(&str, &str)] = &[
     ("Maximize Panel", "最大化面板"),
     ("Media Replacement", "媒體替換"),
     ("Mesh:", "格點："),
+    ("Diameter:", "直徑："),
     ("Method Used: {}    Average Error: {} pixels", "使用方法：{}    平均誤差：{} 像素"),
     ("Models folder: {}", "模型資料夾：{}"),
     ("Modifiers:", "修飾鍵："),
@@ -1757,6 +1760,7 @@ const UKRAINIAN: &[(&str, &str)] = &[
     ("Maximize Panel", "Розгорнути панель"),
     ("Media Replacement", "Заміна матеріалу"),
     ("Mesh:", "Сітка:"),
+    ("Diameter:", "Діаметр:"),
     ("Method Used: {}    Average Error: {} pixels", "Використаний метод: {}    Середня похибка: {} пікселів"),
     ("Models folder: {}", "Тека моделей: {}"),
     ("Modifiers:", "Модифікатори:"),
diff --git a/crates/ui-egui/src/panels/roto_tool.rs b/crates/ui-egui/src/panels/roto_tool.rs
index f9090981..fbd7119c 100644
--- a/crates/ui-egui/src/panels/roto_tool.rs
+++ b/crates/ui-egui/src/panels/roto_tool.rs
@@ -168,6 +168,32 @@ pub fn interact(
         let sign = if kind == "bg" || kind == "refineErase" { "−" } else { "+" };
         painter.text(hp, Align2::CENTER_CENTER, sign, Tokens::semibold(12.0), col);
     }
+    // ⌘/Ctrl-drag resizes the brush (After Effects): the circle stays at the press point and its
+    // edge follows the pointer. No stroke is painted.
+    let rid = egui::Id::new("layer-panel-roto-resize");
+    if resp.drag_started()
+        && mods.command
+        && let Some(pos) = ui.input(|i| i.pointer.press_origin()).or(resp.interact_pointer_pos())
+    {
+        ctx.data_mut(|d| d.insert_temp(rid, pos));
+    }
+    if let Some(origin) = ctx.data(|d| d.get_temp::<Pos2>(rid)) {
+        if let Some(pos) = resp.interact_pointer_pos()
+            && zoom > 0.0
+        {
+            let dia = (2.0 * f64::from(origin.distance(pos)) / zoom).clamp(1.0, 5000.0).round();
+            let key = if refine { "refineDiameter" } else { "diameter" };
+            if let Err(e) = app.session.execute("roto.options", json!({key: dia})) {
+                app.ui.status = e.to_string();
+            }
+            painter.circle_stroke(origin, (dia * 0.5 * zoom) as f32, Stroke::new(1.5, col));
+            painter.text(origin + vec2(0.0, -14.0), Align2::CENTER_BOTTOM, format!("{dia:.0} px"), Tokens::ui(11.0), col);
+        }
+        if resp.drag_stopped() || !resp.dragged() {
+            ctx.data_mut(|d| d.remove::<Pos2>(rid));
+        }
+        return true;
+    }
     let did = egui::Id::new("layer-panel-roto-drag");
     if resp.drag_started()
         && let Some(pos) = ui.input(|i| i.pointer.press_origin()).or(resp.interact_pointer_pos())
diff --git a/crates/ui-egui/tests/ui_viewer.rs b/crates/ui-egui/tests/ui_viewer.rs
index fd12d555..bada77a7 100644
--- a/crates/ui-egui/tests/ui_viewer.rs
+++ b/crates/ui-egui/tests/ui_viewer.rs
@@ -1837,3 +1837,40 @@ fn full_resolution_frames_wider_than_the_texture_limit_fit() {
     assert!(full(&mut h), "the full-size frame is shown (and stays readable): {:?}", h.state().ui.viewer.res);
     assert_eq!(h.state().viewer_texture_size(), Some([800, 134]), "2400×400 averaged by 3");
 }
+
+/// #509: ⌘/Ctrl-drag in the Layer panel resizes the Roto Brush (as in After Effects) without
+/// painting, the next stroke uses the new size, and the tool options show the Diameter.
+#[test]
+fn roto_brush_ctrl_drag_resizes_the_brush() {
+    let mut h = harness();
+    let id = h.state().session.active_comp().unwrap().layers.iter().find(|l| l.name == "Box").unwrap().id.0;
+    h.state_mut().session.execute("layer.select", json!({"layers": [id]})).unwrap();
+    h.state_mut().ui.layer_panel = Some(id);
+    h.state_mut().show_panel(effectcraft_ui_egui::dock::PanelKind::Layer);
+    h.state_mut().ui.tool = Tool::RotoBrush;
+    h.run_steps(3);
+    assert!(h.state().auto.find("header.roto.diameter").is_some(), "the Diameter field");
+    let canvas = rect(&h, "layerPanel.canvas");
+    let zoom = canvas.width() / 80.0;
+    let c = canvas.center();
+    h.input_mut().events.push(Event::ModifiersChanged(egui::Modifiers::COMMAND));
+    drag(&mut h, c, c + vec2(20.0 * zoom, 0.0));
+    h.input_mut().events.push(Event::ModifiersChanged(egui::Modifiers::NONE));
+    h.run_steps(2);
+    let dia = h.state().session.state.roto.diameter;
+    assert!((dia - 40.0).abs() <= 1.0, "a 20 px drag makes a 40 px brush: {dia}");
+    let strokes = |h: &Harness<'_, EffectcraftApp>| {
+        let l = h.state().session.active_comp().unwrap().layer(LayerId(id)).unwrap().clone();
+        let roto = effectcraft_engine::effects::roto::ID;
+        let g = l
+            .effects()
+            .and_then(|fx| fx.groups().find(|g| matches!(&g.kind, effectcraft_engine::project::GroupKind::Effect { effect } if effect == roto)).cloned());
+        g.map(|g| effectcraft_engine::effects::roto::data(&effectcraft_engine::effects::flatten_params(&g, &mut |p| p.value.clone())).strokes.clone())
+            .unwrap_or_default()
+    };
+    assert!(strokes(&h).is_empty(), "resizing paints nothing");
+    drag(&mut h, c - vec2(10.0 * zoom, 0.0), c + vec2(10.0 * zoom, 0.0));
+    let s = strokes(&h);
+    assert_eq!(s.len(), 1);
+    assert!((s[0].radius - dia / 2.0).abs() < 1e-6, "{} vs {dia}", s[0].radius);
+}