Many snowshoe manufactures publish weight ranges for their snowshoe models. If you weigh this much – then use this size snowshoe. That is helpful to some extent. It can eliminate a few models that you won’t have to consider but the weight chart doesn’t tell the whole sizing story.
Figure out what what kind of snow you will be in most often. For instance when I snowshoe I go right out my kitchen door and hike up the hill behind my house. I use the same trail pretty much everyday. The only snow that I’m worried about breaking a trail through is the snow that has fallen overnight. I don’t need a big decked snowshoe. I can use a smaller deck to save weight and increase manuverability.
But the next guy who weighs the same as me and is using his snowshoes up on the pass with a big pack and breaking a fresh trail everytime he goes may want the largest deck he can find to increase floatation. We weigh the same but our priorities are different.
Which snowshoe size is best for you? Don’t just consider weight but consider where you will be snowshoeing most often. That is really the deciding factor.