Borrow and test as many skimboards as possible. Not many shops rent skimboards, but it's easy to make one. Test different size skimboards.
Bigger boards work better for bigger people.
Realize that larger surfboard-shaped boards are better for wave riding and smaller, rounder boards are better for tricks. Note that a skimboard is easily shared and passed around on the beach, so a middle size is often your best option. Lay the skimboard flat on the ground in the shop. Ideally, the board will sit flat on the ground with the nose and the tail slightly tilted upward. Compare different bottoms on boards.