I am planning on getting a new muni and don’t know what size to get. I am currently riding a 20" torker. If i get a 24 inch will there be that much of a difference or should i look at the 26 inch. I am planning on riding this on road and off road.
These are the two I am looking at.
http://www.unicycle.com/unicycles/mountain-unicycle/nimbus-24-inch-muni-cycle-3.html/
http://www.unicycle.com/unicycles/mountain-unicycle/nimbus-26-inch-unicycle.html/
Also i might have to upgrade the tire to an 85 psi one, I am around 260 lbs. Will both of these be able to do it?
Not quite the leap you want to make but I recently upgraded from a 24" to a 26" and didn’t notice much of a change. At first I felt like the 26" I was riding too high, but I quickly got used to it. What are your goals? Distance/touring? If so I’d probably recommend the 26" over the 24".
When I was deciding on my next uni I was torn between a 26" and a 29" I decided that the 29" might be difficult/intimidating so I got the 26". After quickly getting used to the 26" I kind of wish I held out for the 29". Just putting this out there if you are interested in distance/speed. I’ll eventually get my 29" uni and end up with 4-5 unis to pick from one day.
Also, for off-road you’ll want to go low pressure. I’ve heard suggestions on this board that recommends a PSI of [your body weight]/10. So you might ride at 26 PSI off-road. And yeah, I pump the tire up for road riding - I’m 180 and ride at about 60 PSI, depending on the tire.
I’m thinking i want to get more into muni. Now the 26" one has a 125mm crank. Isn’t that on the short side? Considering that the 24 has a 150mm one stock.
Yeah, I got my 26" to do muni. I though 165mm cranks were standard on 26" munis. I’m currently on a 26" with 150mm, and it’s a nice challenge. I don’t think you are going to want 125mm cranks on a 26" muni. You are going to want the longer cranks for muni just for that extra leverage for obstacles.
I would make my decision between the two wheelsizes based on the following:
if you want to ride with a 3" tire take the 24" wheel
if you want to ride with a smaller tire, like 2.5", take the 26"
Of course you could ride a 26" with fat tires (3" and up) but I personally think that this makes the uni too heavy for technical trails. I prefer a 26" wheel with 2.5" tire because you have a huge choice of replacement parts from the biking world which you can get almost everywhere (if you ever tried to get spokes for a 24" wheel in you local bike shops you might know what I am talking about 
). Such a setup can even be lighter than a 24x3 setup.
For Muni I also would recommmend 150mm or longer cranks and a brake if you can afford it (a brake is not only useful for slowing down but it also helps on technical sections even if they are not too steep and your knees will thank you when riding down roads)
That’s because a fat 24" tire is often about the same size as a not-so-fat 26" tire. Based on that, the speed difference is very minor. Instead think about the performance you want. The smaller wheel will tend to be more bomb-proof, though this depends entirely on the parts. As both of your examples are about the same price, expect a somewhat stronger wheel on the 24" (all other things being equal, smaller is stronger). The 26" shown is a hybrid, so it will work better for a mix of rough terrain and pavement.
I went from a 24" 1.7 to a 26" 3.0, so it was a decent upgrade for me.