Replace width and height with Widget::size

widget/src/space.rs

@@ -45,12 +45,11 @@ impl<Message, Renderer> Widget<Message, Renderer> for Space
 where
     Renderer: core::Renderer,
 {
-    fn width(&self) -> Length {
-        self.width
-    }
-
-    fn height(&self) -> Length {
-        self.height
+    fn size(&self) -> Size<Length> {
+        Size {
+            width: self.width,
+            height: self.height,
+        }
     }
 
     fn layout(