Examine the paperwork that came with your bike. Find the information specific to bike tire size.
Visit the manufacturer's website. Typically if you provide the model number for your bike, the site will have information on the standard tire sizes.
Look at the sidewall of the bicycle tire. Many tires contain an imprint of a number pairing, such as 700 x 26. The first number is the approximate diameter of your tire in millimeters, while the second number corresponds to the tire width in millimeters. Additionally, determining the type of tire on your bike will help you better understand the numbers.
If you cannot find numbers on the wheel, you will need to measure the rim yourself. The best way to do this is to measure the tire from rim to rim or axle to rim. Take the number and double it. This will give you the approximate circumference measurement.
Using a wrench, loosen the bike tire from the wheel housing. Take the wheel to a bike shop in your area. Employees at a reputable bike shop will be able to tell you the wheel size just by looking at the tire.