Good question, do you have to have a 21' boat, no. All you have to do is meet the tournament rules minimum boat length and motor size.
What I would recommend is to make a pro vs con list of everything you'll face as a boater. Remember, as a boater you will have to be prepared for all types of conditions.
Some things to think about as you make your choice:
Water conditions; rough (storms) / calm
Fuel capacity; how far will your runs be
Partner; size and how much gear will they bring
On and on
It's going to be a tough decision. Just make sure you don't regret that decision because your making a big investment.
Good luck