See the question and my original answer on StackOverflow

By default, elements have an HorizontalAligment and VerticalAligment properties that are set to Stretch, so they will adapt to their parent.

But if you set these to Center, the element will keep its natural size, so you can test this (or setting properties using code instead of XAML):

<TextBlock Text="This is my text"

It worked in my case, maybe not in every case.