As everyone said. Ask yourself if yout willing to leave your boat outside or garaged. If you decided to garage your boat who deep is your garage and how many people are you willing to fish with. I had a 17 foot Nitro but bought a 19 footer a year ago after my family grew. I spend most of the time fishing by myself however when I bring the family its nice to have the extra room. Storage space is another thing to consider. How long are your rods? My 17 foot boat could not hold anythig longer that 7 foot. My 19 foot alows me to almost drop those rods right in.
Ask about a swing away toung. This is important when garaging your boat. How long is the boat from prop to tip the trailer. Will you be fishing Diamond Valley? Ask if your boat has an EPA approved motor.
The list can go on and on. It depends on your needs and how much your willing to spend.