Size preferredSize

The size that this RenderCustomPaint should aim for, given the layout constraints, if there is no child.

Defaults to Size.zero.

If there's a child, this is ignored, and the size of the child is used instead.

Source 

Size get preferredSize => _preferredSize;
void preferredSize=(Size value)

Source 

set preferredSize (Size value) {
  assert(value != null);
  if (preferredSize == value)
    return;
  _preferredSize = value;
  markNeedsLayout();
}