To reiterate what John said, you shouldn’t worry about your unicycle for now. When you’re trying to get from an 80cm hop to a 100+ cm hop, only then should you start worrying.
To be fair, I’ve seen riders make 100+ cm hops on 24" unicycles. This should tell you how little the difference between a 20" and a 24" unicycle makes in the hands of a good rider. You, too can get this good… for now, you just need to practice.
Unless you’ve got pistons for legs, there are other ways to pull your legs up without worrying about clearance. Search “tuck” on the forums, or read through the Trials Discussion Thread for more tips…