Forget about all those exotic tricks you see in the street / flatland videos. Basic building blocks of technique are most easily learned on a smaller wheel. And frequently not learned at all on the larger wheel. Unicycling is not practical. Most of us learned, I’m guessing, because it is weird or because we enjoy a challenge. So, embrace the weirdness, embrace the challenge, get a 20" and learn new stuff.
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Agree. I learned hopping on a 20". With a little time, I was able to transfer the skill to a 24". Both of these I am very confident with. I’m still hit and miss on it on my 27.5, I have to really think about it.
And for me, idling is not difficult at all on the 20". It is a lot harder on bigger wheels. But knowing how to do it on a 20 means I do have the basic ability and if I want to be able to do it on a bigger wheel, at least i have the foundation knowledge. It isn’t easy to do on a bigger wheel though!
Also, going down a kerb etc… it’s not so scary for learners if you aren’t so high up, the possible fall is from not so high 
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Emphasis on “for the teacher.”
Hm, I found the opposite. On the 20, I felt myself idling too fast and trying to just kind of make my body stay in one place, which made me fall. On the 26, which I did eventually “learn” to idle on, I realized the motion that was required - you actually have to move a bit when you’re on a bigger wheel, as trying to stay forwards/backwards while going the opposite way is an instant UPD. Having gotten that part of it, I can now idle on a 20" with not much of an issue (though, admittedly, I am still slightly more comfortable on the 26er). But still, I found the movements difficult to properly conceptualize when I was trying to learn on the smaller wheel.
20" is good for unicycle hockey 
BR Sanne