Try both a Glock 21 and a Glock 30 before you make your decision. The smaller Glocks (anything smaller than full sized) do not fit my hand as well as the larger ones. And for home defense I'd much rather have a G-21. Its bigger, has a longer barrel (slightly increased bullet speed), better sight radius, better grip and holds 3 more rounds.
As for which caliber of Glock to buy, my first choice is .45. I like the .45 cal round better than anything else currently on the market. Tests show that it expands to a larger size than any of other calibers on impact. .45 works so well that it even performs fairly well when using standard FMJ and JHP ammo (which means that you don't need expensive and hard to come by ammo to defend yourself). The best part is that I can shoot .45 a little more accurately than the other calibers which I have tried.
If you don't want a big caliber, then try a 9mm. .45 and 9mm are the only two pistol calibers that you will ever need. I suspect that the other calibers were designed by politically correct cops who didn't like the idea of civillians using a military round.
Keep your supply lines simple, stick with .45 and 9mm and you will never be short on ammo.
-SS