You have heard the saying, Beauty is in the eye of the beholder, right? Size works the same way. But it works on different levels.
I fish a lot of creeks where the trout are on the average of 10 inches. So I'm used to that, but once in awhile I catch what I call the gem of the creek which is over the norm, so say a 14 incher. To me, that's a big fish for a few reasons. 1, it's bigger than the average fish in that creek. 2, it could be the biggest fish I have ever caught anywhere, so it's a personal best.
Location plays a big roll too. Take a large mouth bass from Florida for example. I hear catching 10 pound bass is common in the lakes there. But in Colorado, a 5 pound bass is a big fish. Our sunfish and blue gills are not the same size as in Minnesota, so ma sunny the size of your hand here is a giant, but in Minn., consider it normal.
I know people who fish lakes, and they always show me photo's of the large trout they catch, from 5 pounds up to I think the biggest I have recently seen is just over 11 pounds my friend said. I think it looks more like 13 pounds personally, but those are large fish to someone who doesn't catch those sizes all the time. But for those who do, it may be a normal size for them. I once saw a man who docked his boat and he shown me a rainbow that to me was impressive. It had to be 7 pounds, and when I commented on it's size, he just smiled and said that there are bigger ones in there.
So that's my opinion on it. Location and personal best. A large fish can be found anywhere depending on those two things. Some areas of rivers have larger fish than other parts of the same river. So again, location because of the environment of that stretch of river. Then again, it's all a matter of opinion.