I previously said that I was going to buy a cheap used throwaway 20" to learn on, then buy a new 24" Nimbus Muni with the 3" tyre.
However, I have not found a 20" yet and was wondering whether to just go straight for the 24?
What percentage of newbies decide not to bother having tried it?
After all, it is a big investment if I go for the Nimbus straight off and then pack it in? Help with decision needed !!!
I was in same situation. So I did not buy 20" and just learned on my 26" 
Did you find the 26" easy to learn on, and how was the experience?
After learning 26" (I described process here) I tried 20" uni and that was much more difficult for me to ride on…
26" giving you more time to react. And you have to use “horizontal cranks” position for easy free mount (or hold wheel with hand) - this is only difference.
I can say the same thing in reverse. I learned on a cheap 20", and then when I was fairly competent, tried a friends 24" Norco with a fairly small tire. I was expecting it to take a while to figure out, but it was almost instantly easier than the 20". I was not the best at idling on the 20", but could immediately idle on the 24".
I had also heard, and assumed that the 20" would be easiest to learn on, but now I feel that may not have been the case. I probably would have been better off with a 24", and been happier with the size of the uni when I was able to ride. The 20" won’t take any real hopping, so I probably won’t use it much anymore, where I may use the 24". I am 29 and about 185 lbs
I say get the 24", but it’s all a question of how sure you are you will learn. I understand not wanting to spend a lot of money on the first one, just in case you give up or don’t enjoy it. That’s what I did, but I’m generally a cautious buyer. If you feel confident you’ll enjoy it and keep at it, may as well spend the money
I learned on 24. If you get a 24 Muni to learn on, you probably would want to get a road tire also. Not that you can’t learn on the knobby, but a tire makes a big difference in how it handles. I didn’t realize how much until recently when I put a hookworm on my 24 muni, it rides sooooooo much better on the road.
If you buy an expensive unicycle and then don’t bother learning any number of people here will buy it from you for a fraction of what you paid. 
Go for it.
Go for the 24
I bought a cheap uni that was supposed to be a 20, but it turned out to be a 24. If I can learn on it at my age, anyone can.