The crank length : wheelsize ratios determine the “gear” of the unicycle, because the resulting number is proportional to the amount of “leverage” you have against the road. The higher the crank length:wheelsize ratio, the “lower” the gear: it’ll be easier to turn the wheel but you’ll go slower for a given cadence.
However, it’s not the only difference: larger diameter wheels roll better over bumps, and also have more stability due in part to their larger mass. I’d expect you to go faster on a larger wheel given the same theoretical “gear” due to these factors (and because it’s difficult pedalling fast on tiny cranks in my opinion).
For a concrete example:
My friend Alan has a 20" uni with 80mm cranks. I have a 29er with 127mm cranks, and a 36er with 150mm cranks. The highest theoretical “gear” is the 20" uni, but it’s also the slowest of the three. The 29er is substantially (several mph) faster with me riding it, despite my being less fit - I’ll hit, say, 13-14mph where he tops out around 10mph. Alan can take my 29er up to 15mph.
The 36er is faster again, with me topping out at a maximum of 17mph (so far), despite the wide hub, and long high Q cranks that I’ve got installed.
The ability to roll over bumps and be stable at high speeds (and have a decent stroke length when pedalling) makes the bigger wheels much preferable for any distance. However, they are also a bit more unwieldy, harder to turn, harder to thread through busy streets, etc. The 20" can easily go down to walking pace and thread through pedestrians.
For cross country offroad the bigger wheels will roll over bumps better than an equivalently geared smaller wheel (although it is harder to build up the necessary momentum due to their probably being harder to freemount offroad). If you wanted to do technical stuff you might find you need a smaller wheel.
I find the 36" easily best for road riding, and for XC offroad I’m liking it too. A 29er can make a good muni though - I’m going to put 150mm cranks on it for the purposes of muni, unless I’m doing really rough stuff that should roll over most things.