What is easiest wheel size to learn on for an adult?

Got a 29" inseam, am 5’ 10" tall. Would a 24" wheel be easiest to learn on or a 20"?

Also are thick tire mountian unicycle easier to balance and roll on than the thinner tire models?

A 20 or 24 should be fine.

Both are good. If you already have the 29", you could go for the 20" for greater variety, or the 24" for something a little closer to your end vehicle. Those cycles may also get used by you to teach others to ride, so if you have kids in your life, the 20" may be a better fit depending on their size.

Also fat tires are not so great for learning.