The general rule of thumb is that snowshoes should be about 10-15% of your body weight. For example, if you weigh 150 lbs, you would need a pair of snowshoes that weigh between 15-22.5 lbs.
Of course, there are other factors to consider when choosing the right size snowshoes. The type of terrain you'll be hiking in and the amount of snow on the ground are also important.