For your own convenience, I would go with a minimum 5# CO2 tank. Anything smaller will need frequent refilling, which is the biggest pain about gas tanks, IMO. I have a 20# tank for my reactor with a 5# backup tank so I don't have to rush out for a refill.
I have bought several CO2 tanks cheap on craigslist. Get one, then just swap the tank out for a filled one when it is empty, then you never have to worry about paying for a hydrotest every few years. Also, the cost of a swapout for a 5# tank is almost as much as a 20# tank, so get a 10# or 20# tank if you can. Cheaper to operate and they last a lot longer between refills.