How to change the font size of all(n number of ) texblocks inside the stack panel programmatically?

You can apply a style in markup:

<Style TargetType="TextBlock">
  <Setter Property="FontSize" Value="20"/>

Yes, You can refer the code snippet below, where 'foobar' refers to your Stackpanel's Name.

        foreach (var children in foobar.Children)
            (children as TextBlock).FontSize = 20;

If you want all Subelements another Style why not use "ContentControl"?

For example like this:

    <GroupBox Header="Some Header" FontSize="18" FontWeight="Bold">
        <ContentControl FontSize="14" FontWeight="Normal">

All elements inside the ContentControl Block will be st with normal weight and a size of 14.