Question

why are ski-board that are used to glide over snow long and broad

Answer 1

Long and broad skis increase the area of contact of the skier with the ground.This reduces the pressure exerted by the weight of the skier on the ground.This helps the skier to move freely and avoid sinking into the snow.
I hope it helps.....

Answer 2

As the area over which force is applied increases, the pressure decreases. A long and broad plank helps to increase the area over which force due to the person's weight is applied, hence it reduces the pressure over ice and makes it easier to ski as the board does not get stuck on ice.   : )