Hi Jelle!
You can in fact fix this very simple.
In the Size-expression you can change the `h` to `value[1]`
So it would look something like this:
SIZE
s=thisComp.layer("1 - Name LastName");
w=s.sourceRectAtTime().width;
h=s.sourceRectAtTime().height;
[w, value[1]]
POSITION
s=thisComp.layer("1 - Name LastName");
w=s.sourceRectAtTime().width/2;
h=s.sourceRectAtTime().height/2;
l=s.sourceRectAtTime().left;
t=s.sourceRectAtTime().top;
[w+l,h+t]
This will now fix the standard height to the value you set it in After Effects itself.