If it's a small frame, it probably had 26" wheels to begin with. You can probably get 700c or 650b wheels to work. Your main issue would be lining the rim up with the brakes- changing wheel sizes means you might need a longer reach brake. I'd take it to a shop and have them help you figure that part out at least. They should have wheels in those sizes to try and see if they work. Guessing with a frame that small it wasn't meant for 27" wheels anyway.