For shooting 1 ounce loads, I like a 16 or 12 weighing an ounce or three either side of 6 pounds. My two 20s are about 8 ounces lighter than that, and I generally feed them 7/8 oz loads. As LF & Danross point out, the larger bores are ballistically superior, at least in principle, for a 1-oz payload, but even that notion probably falls apart against the variability of borings and patterns. In the end, it comes down to the individual gun, what you want to do with it, how well it fits, and personal preference. Danross's SKB sounds like a a great field gun, but 2.5" chambered light 12s and 16s can be found at pretty reasonable prices, especially in the form of French boxlocks.
It's possible some day I'll find a 6-pound 20 I just can't resist, but I'd be surprised to see it happen. I'd be more likely to be smitten by something quite at the opposite end of the spectrum--like this:
http://www.gunsinternational.com/guns-fo...un_id=100983892 Note that's not 5# 15oz but 5# 1.5oz! For me, that would be a 3/4 oz gun.
