Everyone should have a 24 Standard unicycle. Mine is an LX Torker with Venture 125 cranks (that is the shortest allowed for a Standard Uni). It has a Maxxis DTH 24x1.75. Very nimble and it is a great track uni as well as going pretty much anywhere on hard surfaces without a brake
However I don’t see the point of a 24 off road. They just don’t roll over stuff.
Sounds like you need to decide on what your priority is… I’m of the rare opinion that contrary to popular belief on this forum, owning more unicycles doesn’t make you happier (or is necessary to be a better unicyclist). And if your priority is riding Muni, a 20" beginner uni and a Muni is all you need.
The 26er on trading post is okay as a starter Muni, but I personally think, all things considered it’s simply not as amazing of a deal as Bug72 thinks it is. You’ll likely want a modern seat, it doesn’t have a Diskbrake mount and while I have only made positive experiences with buying used unicycles, it’s still buying something used sight unseen. Factor that in and a new steel framed Nimbus Muni is suddenly competitive I’d say. Not saying it would be a bad buy, but it mainly depends on how soon you would like/need a diskbrake.
Yes, be prepared to have an adjustment period when changing from a 20" to 26"/27.5". For some people that takes longer, for some people it’s slower, just like learning to ride a unicycle.
HI dan,
Just thought i would throw my sugestions out there.
I went from 20" first unicycle to a nimbus 26" and yes it took some time to used to it but not too bad. Since then i have had various sizes.
I had a 24" but got rid of it as i found it slow to ride distance on and a strange size to learn anything on even thou i did kind of like it.
I have recently built a 27.5" muni and have managed to keep it light in weight and am running it tubeless, I find the uni has nice speed to it for riding distance and being light it is very nimble and i can throw it around easily enough. I still ride my 26" nimbus but it is heavy being inner tubed, steel frame etc but size wise i find there is no real differnce between 26" & 27.5"
my sugestion would be go for the 27.5" over the 26"
I was wanting a 26" over a 27,5. But thats because I already have a 29". Now im thinking its not worth the money.
Ya, my priority is what you just said. A muni and my current 20" beater.
I was looking at the Qu-ax on renegade juggling. A new steel 27.5" qu-ax muni without a brake is not much more then the used 26er with out a brake ($290 vs $250). Though, I may be able to haggle a bit more on the used 26er. Anyway, the seller hasn’t responded yet. Either one of those would be another $100 for a magura brake system. Disc brakes are nice when window shopping the internet but not necessarily a requirement for me. Though, I do know I’d like to have brakes even if I’m not at the stage of needing them yet. I will be. Magura brakes on a new quax might not be a bad deal now that I’m thinking about it. $390 plus shipping. hmmmmm 
You can do the maguras but I think you should be able to do disc brakes though for the $100 mark? Assuming you are trying to attach it to a decent uni.
And put it all together.
I want your local bike shop.
You forgot rebuilding the wheel in order to fit a hub that has a disk mounts, or adding KH Spirit cranks on the Nimbus. (Either of those will push you well over the 100$ mark…)
I’ve been relearning unicycling after a 35 year break and you can learn on a bigger wheel. I figured out how to freemount and hop on a 36er. Surprisingly, it wasn’t super easy to freemount the 27.5 afterwards. Hopping on the 27.5 was though.
It may take you a little longer, but your eventual goal is to be doing these things on the muni anyway, right? I’m not saying don’t get the trials uni, but don’t think its a necessity either.
If ypu happen to have your wheel built with a disc hub already… easy. Or yes, go the spirit cranks. Not cheap, but you also get a dual crank length benefit out of it, and its a 2 in 1 win.
Mine had a whole box load of adaptors, they were happy to help me. You just had to select the correct ones to use for the setup.
I guess they are leftovers from bike builds or repairs.
I think a 26 is a little easier than a 27.5, and I also get less tired. If it’s fairly easy terrain, I’d go with the 27.5. If it’s more technical, I’d probably choose a 26. Maybe it’s just me, but I feel closer to the ground and therefore safer on a 26. Even if there are more tyres available for 27.5, there are still options for 26. I use a Surly Dirt Wizard 26X2.75 which I like a lot.
Regarding disc brake vs rim brake (Magura). Disc brakes are better, but the difference is less with a 26/27.5 than on a 36 (or if you have a geared hub). Personally I wouldn’t replace a rim brake on a 26 with a disc brake, but I use long cranks (150) for muni which gives me more control. Magura rim brakes have been more durable than the disc brakes I’ve had.