diff --git a/decorations/decoratedclient.h b/decorations/decoratedclient.h
--- a/decorations/decoratedclient.h
+++ b/decorations/decoratedclient.h
@@ -66,6 +66,7 @@
     QPalette palette() const override;
     QColor color(KDecoration2::ColorGroup group, KDecoration2::ColorRole role) const override;
     bool providesContextHelp() const override;
+    QSize size() const override;
     int width() const override;
     WId windowId() const override;
 
diff --git a/decorations/decoratedclient.cpp b/decorations/decoratedclient.cpp
--- a/decorations/decoratedclient.cpp
+++ b/decorations/decoratedclient.cpp
@@ -67,6 +67,7 @@
             if (oldSize.height() != m_clientSize.height()) {
                 emit decoratedClient->heightChanged(m_clientSize.height());
             }
+            emit decoratedClient->sizeChanged(m_clientSize);
         }
     );
     connect(client, &AbstractClient::desktopChanged, this,
@@ -273,6 +274,11 @@
     return m_clientSize.height();
 }
 
+QSize DecoratedClientImpl::size() const
+{
+    return m_clientSize;
+}
+
 bool DecoratedClientImpl::isMaximizedVertically() const
 {
     return m_client->requestedMaximizeMode() & MaximizeVertical;
diff --git a/kcmkwin/kwindecoration/declarative-plugin/previewclient.h b/kcmkwin/kwindecoration/declarative-plugin/previewclient.h
--- a/kcmkwin/kwindecoration/declarative-plugin/previewclient.h
+++ b/kcmkwin/kwindecoration/declarative-plugin/previewclient.h
@@ -90,6 +90,7 @@
 
     int width() const override;
     int height() const override;
+    QSize size() const override;
     QPalette palette() const override;
     QColor color(ColorGroup group, ColorRole role) const override;
     Qt::Edges adjacentScreenEdges() const override;
diff --git a/kcmkwin/kwindecoration/declarative-plugin/previewclient.cpp b/kcmkwin/kwindecoration/declarative-plugin/previewclient.cpp
--- a/kcmkwin/kwindecoration/declarative-plugin/previewclient.cpp
+++ b/kcmkwin/kwindecoration/declarative-plugin/previewclient.cpp
@@ -111,6 +111,11 @@
     connect(this, &PreviewClient::bordersLeftEdgeChanged,   this, emitEdgesChanged);
     connect(this, &PreviewClient::bordersRightEdgeChanged,  this, emitEdgesChanged);
     connect(this, &PreviewClient::bordersBottomEdgeChanged, this, emitEdgesChanged);
+    auto emitSizeChanged = [this, c]() {
+        emit c->sizeChanged(c->size());
+    };
+    connect(this, &PreviewClient::widthChanged, this, emitSizeChanged);
+    connect(this, &PreviewClient::heightChanged, this, emitSizeChanged);
 
     qApp->installEventFilter(this);
 }
@@ -133,6 +138,11 @@
     return m_height;
 }
 
+QSize PreviewClient::size() const
+{
+    return QSize(m_width, m_height);
+}
+
 QString PreviewClient::caption() const
 {
     return m_caption;