A skateboard has 4 wheels.
Wheels dont make a skateboard easier to turn on the skateboard trucks do that.
the friction between the wheels and the surface of the ground affect the speed of the skateboard. fo instance if you ride the skateboard on a smooth surface (with less friction) the skateboard will go faster.
it goes faster
yes the sizes of skateboard wheels range from 40-70 mm
For a skateboard, friction works between the wheels of the board and the axis, as well as between the wheels of the board and the ground when the board is in motion. Since friction works in a direction opposing motion, it is only in effect when the when the board or the wheels of the board are in motion.
It is pretty much a wheely on a skateboard but with 2 wheels.