The unique design allows your Mongoose to form to your hand after a brief breaking in period. Designed for a consistant roll and better lift through proper thumb release, and to provide proper wrist support. Keeps the little finger tucked in as recommended by many touring pros.
How do you know size?
Unfortunately Mongoose does not have a size chart to check sizing. However, it seems to be close to the other wrist supports so if you have a previous wrist support you could probably go with that same size and be close.