Long vs Short

A snowboard should typically be as tall as your chin is from the ground, but sometimes people like them either a little shorter or a little taller. The shorter your snowboard is, the slower you will go down the mountain; thus the longer your snowboard is, the faster you will go down the mountain.
A smaller snowboard would be more preferable in the terrain park because it's easier to control when grinding on rails and going off jumps. On the other hand, a longer snowboard would be more preferable when "cruising" down the mountain. A longer snowboard not only gives you more speed, but it's also easier to control, it gives you a smoother ride, and you are less likely to "bite an edge". Biting an edge would be when your board digs into the snow, making you fall suddenly and very hard to say the least.
The size of a snowboard is completely up to your personal preference, some boards just handle better on different terrains.