What size wheel should i get for going off-road unicycling. I’m 5’4" and have been riding for about 4 years.
24
filler
go for a 24" that way it can roll over objects easier and can climb easier and it wont be to big for ya…alternatively you could do the 20" thing but it wont climb or roll over nearly as easy…my recomendation 24"
I’d say 24. I did my first MUni on a 24, and I would have hated it if I was on a 20. I think possibly a 26 might be ok too, but I’m not too sure there. But 24 is the most common sort, so that must say something for it too 
24…
Very few people use a pure 24" for muni- it’s generally a 24x3 or similar ie a 24" uni with a fat tyre.
I think it’s important to make the distinction, as it’s often relative newbies who ask these questions, and it’d be a shame if they bought a 24"-er with a 1.75" tyre and expected it to be good for muni.
A 24x3 is superb for rolling over rough terrain and has a effective diameter close to that of a 26" wheel. I think it would be fair to say that the introduction of 3" tyres for the 24"-er, revolutionised muni- to the extent that most experienced unicyclists, when they read ‘24" uni’ automatically assume it’s a 24x3; but, a lot of newbies, wouldn’t realise that.
(obviously, in this case, it’s not a newbie asking the question, but a lot of people who are new look over these threads to get advice about their new unis).
See the current thread 24/26 for a very similar question.
I ride what I call MUni on a 26 x 2.5", and what I call cross country on a Coker, or sometimes on a 700c x 28 mm (when dry!).
Some people would call what I call cross country serious MUni, but some would call what I call serious MUni cross country.
As the hillbilly said at the school dance, “It’s all relative.”
As you gain experience, you find yourself able to do these things:
Mount and control a big wheel more easily.
Climb and descend hills more easily.
Control a unicycle with short cranks more confidently.
Spin long cranks more quickly.
Only a few absolutes then remain:
Big wheels roll over rough or broken gorund more easily.
Small wheels are more manoeuvreable, and lighter.
A fat tyre makes any unicycle easier to ride on rough or soggy ground.
I now do stuff on a 700c x 28 mm tyre with 102 mm cranks that at one time would have been at the limit of my ability on a 26 inch MUni with 150 mm cranks.
So, if you’ve been riding for 4 years, but you’ve done little or no MUni, where do you start? Well, a 24 x 3 would be a sensible start, but it is not the only answer.