I think the best approach is to create a Configure event callback which rejects sizes that are smaller than you want.

e.g.

'f' ⎕wc 'form' ('event' 'configure' 'onconfig')

⎕cr 'onconfig'
r←onconfig W
⎕←W
:If W[5]<10
:OrIf W[6]<20
r←0
:Else
r←1
:EndIf

This onconfig callback will reject the Configure event by returning 0 if W[5] < 10 or W[6] < 20.

W[5] is Height and W[6] is Width

⎕IO is 1

Regards,

Vince