If the bike has tires now, the size will be stamped on the sidewall of the tire. If it's a small size frame, there's a good chance they're 650 x 23 or 650 x 25. The 650 is the important number. Replacement tires must be this diameter. The other number is the width. The 23 or 25 you have a little leeway. Depending on the wheel, you could go as low as 20 or up to 28 or maybe even 32. But 23 and 25 are probably most common. If the bike doesn't have tires now, just bring it in to your local bike shop and they can set you up.