Each planet is scaled with a minor variance permitted. So as an example, Terran worlds might have a mininum size of 40% of the maximum size a planet can be, with a bonus of perhaps up to 30%. So a Terran would range in size from 40-80% of the maximum size for a planet. Those are not the actual Terran numbers btw. The lower bound is fixed though, so in my example, no Terran would ever be smaller than the 40% size. Rotation speed is set in a similar manner, although frankly Terran planets all seem to be spinning like tops and I'm going to adjust them yet again.