Backcountry snowshoes generally come in three sizes:
8 x 25 inches
9 x 30 inches
10 x 36 inches
The proper size for you depends on two things:
1) your weight (including pack) and 2) snow conditions. See table below.
Note: The sizes may vary by a few inches depending on the brand. Sometimes the shape of a snowshoe will affect the sizing as well, so be sure to consult the manufacturer's own sizing recommendations.