1. How big, or small, of a boat do I need?
Because you want boating to be fun, you will want to purchase a large enough boat to meet all your needs. In the case of a family of four, space will be at a premium. Do you want to entertain guests, or possibly do some cruising? By knowing the primary use of the boat, you can narrow down the size of boat you will need.
In some cases, you may need to sacrifice on the bells and whistles to stay in your budget, yet purchase a large enough boat that has the accommodations you desire. In other cases, you may decide a smaller boat will do just fine, and you can splurge on the bonus items.
2. What is the ideal boat size for the environment I expect to operate in?
It would be fool hardy to daydream about perpetual sunny skies and calm seas when purchasing a boat. It is easy enough to do in Florida, but an entirely different matter in the Puget Sound for instance. Inland lake boating is different than boating on the Great Lakes, which have sea conditions comparable to oceans. When you purchase a boat, take into consideration the size of boat and it's limitations in different environmental conditions.
3. What size boat can I safely handle?
If you are new to boating, although you may want one, a 40 foot boat may not be the best option. That's not to say you can't buy a large boat and learn quickly to captain it well, but for the most part, it would be wise to start small and trade up as your experience grows.
Most people don't want to be in the position of a family I heard about recently. They purchased a 36 foot boat without previous boating experience. After a few close calls, the wife refused to step aboard the boat until her husband takes a course. Since he is a very busy man and taking a course is not an option at the moment, the boat is up for sale. Sadly, they are the perfect example of people starting out with good intentions and getting in over their heads.