The first number you're likely to see on a pair of snowshoes is the width of each individual shoe, expressed in inches. So in a pair of snowshoes that measure 9 by 30, each snowshoe is 9 inches wide.
The second number on your snowshoes is the length. Considering the 9-by-30 snowshoe in our previous example, they would be 30 inches long.
Having snowshoes of the appropriate size means they're big enough to float your weight above the snow but not so large that they make hiking difficult. In addition to their length and width, almost all snowshoes will have a number representing weight range, in pounds, that they're intended to support.
Don't choose snowshoes according to how much you weigh in your underwear; instead, add in the weight of your winter clothes, boots, pack and anything else you might carry in your pack on a snowshoe outing; your snowshoes are going to have to carry all of that.
Women-specific snowshoes tend to be a little smaller than unisex or men's snowshoes. They also tend to be a bit narrower than men's shoes; this is intended to accommodate the relatively narrow gait most women have compared to men.
If you need to carry gear that weighs too much for your snowshoes to float on top of the snow in addition to your body weight, consider pulling the heaviest items behind you on a sled.