If it's your first boat, I would suggest you look for a used aluminum boat. In my opinion it's easier to manage and learn on. They also don't require as big of a tow vehicle. You won't feel guilty about dinging it on a dock, or hitting a stump etc. Aluminum boats also hold their value reasonably well, so if you ever want to upgrade you won't lose much. This will also leave some room in the budget for upgrading electronics like fish finders, trolling motors etc.

As for size, buy as big as you can afford.