I was experimenting with that for some time. I don't think that is possible to fix dimension when you are using zpl II
script instead generate image from QR an scale it to that dimension.
If you won't use QR as image try to manipulate with magnification factor. From doc :
^BQa,b,c,d,e
c = magnification factor
Accepted Values: 1 to 10
Default Value:
1 on 150 dpi printers
2 on 200 dpi printers
3 on 300 dpi printers
6 on 600 dpi printers
So In your case insted :
^BQN,2,10
use
^BQN,2,9
or less.