CBT on a Pit bike anyone?

So I'm getting a pit bike to learn motocross then I'll upgrade after i've been doing it a while. Because I'll be 16 to take the CBT, will I be allowed to ride on a 110cc Pit bike to complete the CBT or do i need a 50cc?. I understand you can have any cc MX bike if you're not using it on the road. My 12 year old cousin has a 125cc so i'm pretty sure it's legal if you're using it on the track. I have been told similar to a car test, if you don't pass manual you have to take a manual test and you can ride manual and automatic. Will i be allowed to do my CBT on a pit bike because they can be made legal or does it need to be a 50cc bike?