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.
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.