Question

Why are rockets different sizes

Answer 1

It depends on where they're going. The Apollo 11 required a lot of fuel, and was therefore bigger because it had to hold it's own fuel. Depending on how far it needs to go, that determines the size.
Hope this helps!

Answer 2

It depends on what there "job" is.

Rockets must be able to accomodate all needed things, as well as have a large enough fuel tank to allow it to fly to it's destination (and fly back most of the time)

If the load is smaller, they would just use a smaller rocket, as it is much more cheaper to produce a smaller than a larger

hope this helps