Hard to isolate lop from other factors/characteristics of gunfit and dynamics. That said, Stan's comments are spot on in my experience. I think the rule of thumb for lop - shoot a gun as long as possible that doesn't hang up or seem awkward in mounting - is pretty good.
I'm built like an ape - 5'9" on a good day, 34" sleeve - am fit perfectly with a straight grip double trigger gun with a 15 3/8" LOP, and 15 1/2" doesn't bother me a bit. With a straight grip single trigger, slightly shorter, but not much. A pistol grip gun moves my preference to 14 1/2" or so.
Keep in mind that this fit business is a strange alchemy - a mix of art and science. Feel of the gun is important, and I am comewhat perplexed that in my stable of shotguns of all types is quite a variety of dimensions, when measured objectively, but the result is the same - great fit on each that I wouldn't change. They all shoot where I look, and somewhat defy logic. But the generalizations I make earlier lean the right direction, I think.
