I doubt it will make much difference in how the cart travels. It will make it easier for the horse to pull, but bigger wheels are also heavier, so it can be a tradeoff. As far as the size of the horse, the cart shafts should be level or uphill, but never downhill.
Miss Myrna's point are all good.
In the end the perceived benefits of changing wheel size by two inches are at best negligible.
When we were first playing around with one cart, we had both a "race" wheel and a practice wheel.
The race wheel was hand built and laced and was a featherweight compared to the practice wheel. We really saw no difference between the two as far as ease of pull for the horse. It seemed that when the added weight was in the wheel itself it did not present the same weight "drag" as when the added weight was in the rig.
Imo, of greater benefit to the horse would be to try and use a tire that has better flotation. In other words, rides on top of the ground vs. carving its way through the ground. This is the easiest way to reduce drag for the animal. This is a good cheater way for a 24" wheel to pretend it's a mighty 26" wheel.....
Bb