Don't just look at boat length though, check out the other details, boat weights vary greatly. The beam size and how much fuel/water/etc weight makes a difference too. Also the trailer weight. If you get a newer boat, unless it's a high end boat, it will weigh less than the same boat that is older due to material differences. Also, if the boat you are looking at has outboards, be careful that the specs on the boat do not include the weight of the motors because they can vary depending on what you put on.
Good luck, hope you find a boat you like that is within your current towing ability. It's a tough choice to replace a truck because you want a boat you can't tow