Rank Upgrade Information

Hello. I am wanting to upgrade from Learner to Builder. I am looking at the challenge board right now, and I am going to try to do a pipeline cpu with ram, stack, function calls, and a good IO. The thing is, I don't know how many bits I need the cpu to be. I want to work on the ALU, but I need to know how many bits to do first. Is there a specific amount needed, or can I just do any amount?