Racing go-karts are fun to race.
I love riding go-karts, which is an open-wheel car.
Once I rode some that looked like bathtubs. 
That's hilarious. Go-karts come in all shapes and forms.
It does make it more fun to try different ones.
Superkarts are supercharged Go-karts that can beat racing cars.
I haven't heard of that. Where can I find some to try out?
There is a racing league. Google it to find the closest location to you. They are just like racing vehicles.
Are they similar in size to go-carts or closer to the size of a small car?
Similar to go-carts. There are regulations to make sure they are not too large.