I use the following to determine the size of a monohull boat I think is good enough and the rules for evaluating other boats.
Let’s say I pick a 45-foot boat, then I’d expect the beam to be 15 feet (carried one third from the stern to midship), the draft to be at least 5 feet, and the free board to be at least 5 feet (or about a tenth the length of the waterline!). Basically, it’s the rule of thirds. In reverse if I have a boat that must fit on an American trailer the boat can’t be wider than 8 feet so should be no longer than 24 feet and have a draft around 2 feet (not counting a swing keel).
Using the above formulation another example can be derived. For open sea work you want the freeboard to be the height of most working waves and to be at least one third the size of a rogue or storm waves. Similarly in construction of glass boats the free board should be a third thicker for every third of the way to the waterline. A two-inch-thick hull at the cap rail would be a four-inch-thick hull at the water line to keel (now that is beefy).
For sea kindly rig you don’t want the rig to be much more than a third longer than the length over all the boat. So, a 40-foot boat could carry a 60-foot rig.
If you take all these numbers determine ballast at between a third and half the weight of the boat all in. Then you start coming up with volume and internal weight numbers that are consistent. That is where design starts to diverge and why all boats don’t look alike. As such there are lots and lots of examples of boats that simply don’t match (like nearly every performance race boat).
Given all that, Uffa Fox supposedly said a man needs a boat with a foot of waterline for every year of his age. I think it better said a man needs a boat with a foot of waterline for every year of the age he acts. I’m going to keep acting like a 40 year old reprobate as long as I can.