I just started riding about 6 weeks ago after about 20 years. When I rode back in the day we didn’t even think to try the trials type stuff that is being done now. I was surprised at how easy hopping was for me. The first day I got my current uni (a Nimbus 24" Muni), I was able to hop indefinitely. This was cool. But I have been practicing alot and still can’t seem to hop any higher than about 8". I haven’t tried seat out hopping yet. Is that what is necessary to get the higher heights? HELP 
24in. are alot harder to jump on than a 20in. Try puting you seat lower, or learn how to hop with seat out in front.
you can jump higher without going to seat out. lowering the seat is one good idea. another is to lower the pressure in your tube (assuming you’ve got it at max psi). also, practice snapping the tire when jumping. if you dont know what that means, basically the ideal method is to push down extra hard as you jump and the bounce of the tire will lift you higher in the air. when i was riding trials on my 24" i could get 17" seat in before i started working on seat out.
i dont know your hopping ability, but practice doing brief stillstands between each hop (maybe 1 second each). this is essential for hopping higher because it will greatly help in setting up for hops.
i will also briefly mention air pressure in your tube. different people prefer different pressures so test it out for yourself. keep in mind, if you can bottom out (rim touches ground) from hopping in place then you are too low. generally speaking, lower pressure is more bouncy and will help if you are just learning trials while higher pressure is needed for really high hops (>28") and for natural trials where you will be hopping on surfaces that are not flat. you should be able to find a suitable pressure for yourself.
edit: if you have U2, watch kh’s section and the part where he is hopping up the rocks at the water park (or whatever that place was). at one point he wedges between the blue tarp and a rock and the camera shifts to a close up where he hops from the rock onto another. in doing so he excersizes the perfect snap of the tire. it is exhaggerated and will help you to see what i mean.
For good tutorial vids see http://unicycle.2ya.com/
Click on “trials”, then scroll down to the KH tutorials, and select basic hopping. Excellent stuff.
Is a 20" uni significantly easier for hopping?
noticeably, but i wouldnt necersarily say significantly. it would make a few inch difference.
defintiley learn to hop with the seat in front