Yes, both in physical size and hp.
about as big as they always was
The YZ is a motocross bike, the trail or woods bike is a WR426.
A cr250 is a motocross bike
By dirt bike do you mean and enduro bike or a motocross bike or some other bike at all? For any bike except motocross bike you definitely need license, and for motocross bike you don't, because you can't ride it on the streets at all... You can only use it in motocross tracks and for that you need to have a special permit...
Become enthusiastic about motocross bikes.
Motocross is a big sport?
For professional motocross, the biggest bike that is allowed to be raced is a 450 four-stroke. For local race purposes there is a Vet class that allows any size bike to be raced (usually up to 525 since that's the biggest motocross bike that KTM makes).
2006.
Just don't think about it too much. Just as soon as you reach the end of the turn twist the throttle all the way (shift if needed) and you will make it over. I ride a yz125 (most underpowered big motocross bike) All jumps are small enough for every bike to take. Unless its laracco's leap at Red Bud.
a bmx bike
Dont.
A fast one.