Usually the wheel number is a nominal overall dia with tire, so, depending on your clearances to the fork crown and a seatstay brace and/or the bottom bracket chainstays you can fit anything smaller than stock, and maybe a bit bigger. Even some cheap and easily available 26" wheels might fit well enough to run. You can also run larger width tires with a smaller dia. than stock. I would check the max radius; measure from the front fork dropout center to the fork crown underside, and deduct a good 1/2" min., probably 1" is better and double it to get dia. You also want to know the max axle size that will fit the dropouts, front and rear, and the inside width that you need to fit the hubs into. A smaller hub can be spaced with washers, and the fork legs and rear can be spread just a bit to pop in something wider; no big deal if you dont go too wide.
