How do you know what size motorcycle to buy?

Firstly it depend on what kind of license you have. With a provisional license you are restricted to a motorcycle with 13 hp, and sometimes the engine size is restricted to 125cc.

Other than that what size of motorcycle you wish to own depends only on your personal preferences, what you intend to use it for(a super bike would be slightly impractical for commuting) and how much your willing to spend(insurance, maintenance in addition to initial costs).

Most motorcycles are around 600cc, the mid class. 1000cc+ motorcycles tend to be very powerful and are easily capable of breaking any speed limit.

My advice is to try as many bikes as you can. Most dealers have demo models and will let you try them.