In XAML how to say: default width and height for e.g. TextBox

So I'm coming at WPF from a HTML perspective.

I just want to put a TextBox on my Window like this:

<Grid>
    <TextBox Name="theName" />
</Grid>

Turns out that the TextBox is then HUGE, covers the whole window. (!)

Ok, that's not what I want, but I don't want to define the EXACT size either since I know Height and Width should be flexible, so I try:

<TextBox Name="theName" Width="Auto" Height="Auto"/>

Same thing. So I try:

<TextBox Name="theName" 
    HorizontalAlignment="Stretch" 
    VerticalAlignment="Stretch"/>

Same thing. So I just hard code the sizes:

<TextBox Name="theName" Width="100" Height="20"/>

Which I know is not a good programming practice in WPF.

So, what how do you tell TextBox to "display default sizes for the font size being used"?