how to make pushbutton in fixed size inside layouted group box

hi friends,
i am designed a group box inside four pushbuttons and i layout the group box .
actually i want to keep the pushbutton size (150 X 50) so inside i adjust the size but after i layout the group box it resized to default so please anybody help me to make the pushbutton constant size...
thank you

Set minimum and maximum size properties to the same value.

thank you very much ... i made it...