I agree it is relative to the fishery. A 20" steelhead or 20" striper is a let down when you’re hoping for a 20 pounder. A 18" rainbow on a lake is no biggie when a 10 pounder is what you’re searching for. A 10 incher can be a small creek lunker. I guess that’s one of the fun things about fly fishing.
Greg
Not sure if I missed something here but I thought a “big” fish is relative to the weight of your rod ie. an 18"er on a 2 weight rod could be considered a whale- regardless of the fishery you stalk.
My $.02
God Blesses!
A wing & a Prayer! ----*<(((><~ ~ ~ ~
Quinn
Night Angler,
That fish is an absolute PIG!!!
Chris, weren’t you posting about how worried you are that the pirates will come up to your home waters and stay? Then you go posting pictures like the one a few posts above. I sense a lack of consistency on your part… 
As far as big fish are concerned, I’m afraid that I can’t speak. I have friends with aquariums who have talked to me about bringing them some of the 2-3" sunfish and golden shiners that I catch, when I’m lucky… 
I guess that a big fish is one that can get a size 12 dry far enough into his mouth to get hooked…
Either that, or these pesky 30+ pound carp that we have all over the place…