Since you haven't brewed yet, how do you know what 16 lbs of grain will produce in terms of starting gravity on your system? If I create a stout recipe with 16 lb of grain and a 5 gallon packaging target on my brewing calculator, I come up with about 1.072 OG which should produce a beer in the 7-8% ABV range. Is that not good enough? (I mean, it might not be, but...)
Stout is a very wide ranging style, with sub-categories having anywhere from 4% ABV on up. So it really depends on your expectations and preferences.
And as
@NewJersey says, if you are planning to bottle the beer, just make a smaller batch size and there will be no problem at all cranking up the grain bill and the ABV.
You might feel paralyzed into following 5 gallon recipes at the very beginning so the idea of making a different volume sounds confusing, but that's a limitation that lasts a very short time, so it shouldn't be used to restrict your gear buying choices.