Not sure I fully understand the question - can you clarify? Screenshots, example code, etc.? I don't think the buttons themselves can be adjusted (AFAIK, they're windows common controls with their size being dependent on the screen resolution), but you might want to toy around with the '/ responsefontstyle' attribute - participants need not click on the small button to select a response option, clicking on the response text will achieve the same thing. So enlarging the response text font might help.
Best,
~Dave