A skateboard has 4 wheels.
Wheels dont make a skateboard easier to turn on the skateboard trucks do that.
yes the sizes of skateboard wheels range from 40-70 mm
you could measure. most skateboarding wheels are between 50mm to 58mm although other skateboards, such as longboards, have bigger wheels that are shaped differently.
It is pretty much a wheely on a skateboard but with 2 wheels.