Weight plays an important role when deciding the size of a fin, especially fins you’d be using for a multi-finned board. For tri-fins, the general weight-to-size range would be; Size S (>130 lbs), size M (130 lbs - 170 lbs), size L (170 lbs +). When determining the size of a single fin, weight is down the list for contributing factors but still should be considered to a degree. For example, those on the smaller size might use a single fin only .5" or .75" different than someone on the other side of the scale.