Here's some 18's on 33's. if I wanted a 20" wheel the minimum tire I'd go with would be a 35"

The shorter sidewall makes steering more precise on the road, period. it also makes it rougher, period.
If you want to go mudding ONLY, the big rims and shorter sidewall tires will be fine, won't make much of a difference. if you want to rock crawl, the smaller rims and taller sidewalls will be better. If the tread is skinnier than the rims, be ready to buy replacement wheels, and lots of them.