I'm not sure through mawah
but in the mame.ini (if you don't have an ini file... mame -cc to create it)
resolution = 640x480 <--- think its resolution.. at work and cant double check it.
but a better solution in my mind is if you dont' want it to change... set windows to 640x480 and tell mame it cant change the resolution or bpp (switchres and switchbpp I think... change them to 0)