Chas,
Well, your idea is certainly one way to get started, and depending on your building ability/desire, probably as good a way as any.
What's a good 'size' to start with? One aspect of that is whatever size that will be easy to transport for you. Another aspect is do you plan to compete/race, or just have fun sailing? There can be a big difference in the price and style of boat between a racing boat and a 'fun' boat. Another one of those "depends" things that depend on your wants. One way of making the "which boat" decission is, which one do you like the looks of? Whichever one you 'like' may not be the 'best' sailer in the world, but there are modifications for everything (just like the model airplanes). Also just like the planes, no one has just one boat! Look at your first boat as something to make you familiar with the things you need to know about sailboats and sailing (hey, it makes a good excuse, right?).
Another aspect of the 'size' thingy is that you need to take a look at the places you have to sail. Some ponds/lakes just aren't big enough for huge boats. Some are too big for smaller boats (as in waves/etc.). [I'd love to have a "1/4 scale" boat, but don't have any place to sail one. And probably would have to license the @#$ thing anyway, not even counting buying/building a trailer to haul it! LOL]
So, keeping all the above stuff in mind, which boat do you want?
- 'Doc

PS - All of the above is 'relative', and subject to change to fit existine conditions.