Hi Duncan, I built my first muzzle-loader when I was 14. There was nothing like this forum then, and only a couple people I could ask questions of, and they didn't know too much more than I did. So, it looks like a piece of $#@*(1). And I still have it some 40 years later.
So, there are a lot of people here who are incredibly talented, who have very high expectations of themselves, and really want to bring everyone up to their level. Don't feel that if you can't make that level right out of the gate, that you shouldn't try. Build what you want, ask questions here and go for it. If it shoots where you point it, and looks great in your eyes, that's what matters. There is nothing like doing something to learn about it. Dollars to donuts you'll want to make another anyway. The thing is, I did it at your age, and if I had the kind of information you have access to (more books, internet pictures like the virtual library here, and this forum), you'll do tons better than I. Taylor S. here I thing built his first at your age as well, and did an outstanding job. So can you.
As far as caliber, it depends on what you want to do with the gun. And if you go with a smaller caliber now, you can make a bigger caliber later. Or the other way around. Keep us posted.
(1) that was a four letter word that started with a c, had an r, an a, and a p. Not some other. Pretty serious "spell checking" here. I don't have a problem with that.