Are we talking about 3-speeds, or perhaps 10 speeds like the Super Courses we've been discussing lately?

In any case, there are a few considerations:

Do you need to fit original size, based on restoration goals? If so, it's obvious.

If not, then wants, needs and practicalities rule. The considerations then are frame/wheel hub width, brake length (i.e. reach), brake width (can the brake open enough to install the wheel without deflating or removing a brake shoe), radial clearance such as for fenders you might be adding), and any effect on toe overlap.

On top of these are cost, rim and tire availability, your weight and intended use, shifting system, use of a generator. For some of us cosmetics are important, too.

Specifically, in going from 28" to 26" I first wonder if you have brakes that can reach the new rims?