In fact the UserControl lacks the property 'SizeToContent' that we have in Window.
So the question is:
what's the easiest and right way to simulate SizeToContent=WidthAndHeight behavior on UserControl?
UPD... yeah I know it suppose to get that automatically if no Height and Width defined in the container where you're placing a user control.
But it doesn't work when you placing one userControl with defined sizes, into another with no sizes, and altogether they go inside the container.
it this case your second control will take all the space it can get.