I used to kayak a lot - abet I never had any desire to do class V, and I only did class IV at safer levels (abet how they define classes depends on the system used). Height however is not really a factor in either of the 2 systems. Danger or difficulty is.
The largest falls I have done were 26' (El Salto in Mexico). But they were easy. There was only one even semi dangerous thing about them - your paddle. You kind of had 2 options - javelin your paddle before/as you were going over (give it a good throw so it was out of the way), or tuck it in tight. Our group was taking our time, so we let a group that was in a hurry run through while we were there. One guy did not know his options, and kept his paddle across his body - his body broke the paddle in 2, that had to hurt. The thing about that particular fall is the water came off the top ledge in a crease and it was literally being poured into the pool of water below. As you went over the edge, you were sucked into that crease, completely enveloping you in the literal river of water being poured in a stream over it. You never really felt hitting the pool below - you were completely surrounded and protected by the stream as it broke the surface of the pool. Unfortunately the blades of the paddle were outside that stream. They hit the surface of the pool and stopped. The stream of water returned back to the surface about 10' downstream (in the pool) from it's entry point. So you just bobbed up safely. I have pictures of it - but they are very unimpressive, as you are underwater as you were falling - just a discolored blur.
I also did a 23' foot (2nd largest fall at the Cascadas de Micas in Mexico. The flow was spread out over too large of area. Even though the fall was into a large deep pool (or at least it was at that time), you hit the surface of the water fairly hard. It wanted to knock the wind out of you. I don't know if I have pictures of that one. But that fall is kind of special - seems like 1/2 the Mexican Restaurants in Texas have pictures of it hanging on the wall. Picture of random kayaker doing it. While there are rocks at the bottom, not that the main flow of water does not really hit them. You can (and do) stand on the ledge and make sure you are not going over in the wrong spot. One other disclaimer - river flow makes a huge difference. There are times it is deadly. Many of the restaurant pictures are at much higher flow than this - you can not see any rocks or fern.