How to increase font size in XAML?

Question

how can I increase font of a, let's say, TextBlock? I don't want to have something like this:

<TextBlock FontSize="20">
  text
</TextBlock>

because it won't work correctly when user changes Windows' settings of the controls' font size. Do we have something like +VALUE (eg. +2), similar to HTML?

EDIT
That's what I meant talking about the Windows' settings:

but the answers I received totally satisfies me.

Answer 1

WPF doesn't have the em font size, there alternatives in the answers to this SO

The simplist may be

<ScaleTransform ScaleX="1.2" ScaleY="1.2" />