I like all trout, especially perty ones. Size does matter though.
But you can't put a number on it, its relative. I enjoy native brookie fishing. On a brookie stream, under 5" is a dink, 6" is average, 8" is a "good" fish, 10" is an achievable trophy, 12" is the fish of a season or more. On bigger waters, 8" is a dink, 10-12" an average fish, 13-15" a "good" fish, 16-20" an achievable trophy, and 20+ is the fish of a season.
And size isn't the only factor. Color, difficulty, location, type of take, how you achieved it (dumb luck vs. some sort of skill or figuring out a good approach, etc.). They all play their part. For instance, I'm no purist, I'll use junk flies, nymphs, streamers, etc. Even bait or spinners once or twice a year. But, I remain convinced that if you catch a fish on a dry fly, you are allowed to consider it a full 3" bigger than it actually is. If its caught with bait, subtract 3".