I fish mostly warmwater and this is what I find.... Smallies key in on size, first and foremost. Early in the season, when food is small, anything larger that say, a size 10 is casting practice. Later in the year, you have to match the larger, 3 or 4 inch food or you won't get bit.

Carp are fussier than any trout that swims, they will literally watch your fly, looking for something wrong with it. Sometimes, it almost appears that they are critical of your tying. "Nope, not gonna eat that... That guy didn't built a nice thread head". Almost seems to be that touchy. Part of what makes them fun.

Presentation is important no matter what. Can't catch fish if they aren't there or if your fly is doing something non foody, they'll pack up and leave