need biggger wheels but not sure on drop

hey guys, I got my dually already lowered as did the guy I got it from, so I have no idea what drop it has without getting under to look. the tires are 225/75/16 all the way around. is there a way to know how big I can go with wheels knowing that info? I assume I am ok if I keep the over all diameter of the tire near the same. thanks for any info. I will put a pic up if some one can guess how low it is. thanks, Kenny.