Boat dealers just love the advice:

"Start with a 16-footer, and then move up two feet per year as you gain
experience."

Horsefeathers.

Unless you are *completely* clueless about what you want to do with the boat,
where you want to use it, how many people will ordinarily be aboard, etc,
that's very bad and expensive advice.

Get a boat that suits your needs and, at the very beginning, learn to operate
it properly. That may even involve hiring somebody to teach you, one on one.
Yes, the CG Aux and the Power Squadron courses are fine......but they won't
teach you how to operate your boat. In some of those organizations, a person
can become
a high level instructor with no requirement that ever, even once in a lifetime,
did they set foot on an actual boat. Theory is good, and the safety stuff
doesn't require boating experience to pass along in a basic form.
Don't skip the course work, but don't even thinik it will begin to prepare you
to handle a larger boat than you start off with.

You're more likely to have a "bad experience" that turns you off from boating
by going out in an undersized boat than in
getting a boat that is actually suitable for your needs to begin with (and
taking the time to learn to run it before you just head out to sea).

Running a 16-foot boat for a year or so does a wonderful job of training
you....to run a 16-foot boat.

There is always a learning curve when you step up in size. Might as well run up
that curve for a boat that actually suits your needs.