Taking the Nimbus 24" and 26" Munis as examples (with 3" tyre).
Is the 26" the weaker wheel in terms of eventual spoke failure and /or rim buckling?
I am only asking in regards to flattish trail riding, rolling over logs, the odd pothole and no big jumping?
I am awaiting my 20" learner unicycle, and will be moving to a larger wheel when I can ride reasonably well.
I currently ride a Catrike recumbent trike, and have occasional loose spokes even on it’s 20" wheels.
In theory, the 24" wheel would be stronger. In practice, for the riding you describe, both are more than strong enough; it’s a complete non-issue.
It’s a non issue, wheels don’t break except when an expert rider does something crazy (ten foot drop onto flat) or your mom drives over your uni with her SUV.
You have a long road ahead, uni is hard, so focus on getting the wheel rolling.
Once you get riding, participate in one of the learner’s threads, it can help keep up the motivation and provide learning tips.
Good luck!
Short answer: The wheel with the better wheelbuild is always the stronger one.
All things being equal (which they usually aren’t), the smaller wheel will tend to be stronger because it has a proportionally wider cross section, and for other reasons I don’t know how to explain. I only know that as spoked wheels get bigger, they get more fragile. Ride my 45" wheel off a curb too hard, and you can get a flat spot. That’ll never, ever happen with the Sun DoubleWide on my 24" MUni.
But for your situation, it doesn’t matter a bit. What I’ve recently discovered, in an online search for 24 x 3" tires, is that they seem to be going out of style. 26" seems to be the wave of the future (or present, or for a while now), and that’s what my next MUni will be.