Yellow perch potentially get much bigger than sunfish- they max out at over 20" long- but most adults are much smaller, less than 12". There are many species of sunfish, some larger than others. Bluegill and redear are the largest sunfish; individuals over 12" long and weighing 2 lbs. are fairly common. Other sunfish in your area include green sunfish, pumpkinseed, warmouth, and northern longear sunfish; the first three are usually under 10" long, and the last is usually under 5".
Perch are a schooling species, sunfish are not. Most sunfish are more aggressive than perch. I suggest stocking your tank with either a school of perch OR a group of sunfish, but not both. I think to keep the two together successfully you would need a lot more room. A possible exception is keeping northern longear sunfish with perch. I don't think the two species would bother one another; maybe someone who has tried this combo will chip in.