Smaller pulley on transaxle will make it go faster ofcourse. Probably no smaller then 4 inch pulley, any smaller and will start slipping. If this is the rock crawler you're building, then probably no smaller then 5 inches on the transaxle and go bigger on the engine, that will give you enough grip with speed and crawling. An 8 inch pulley comes to mind for the engine with a 6 inch on the trans would be favorable.