I find that each image often has an optimal size at which it best expresses (in my opinion) what I mean it to say. In general, I think my images most often need to be printed in the 11x14 to 16x20 range for best effect. Some benefit from much larger prints; few do well if printed much smaller.
That said, I would never say that there's a "correct" size for any given image. A lot of this is in the eye of the beholder and I'm sure others don't see/read my prints exactly the same way I do. I agree with what Alain said earlier: you sell what you show. I've made gallery prints as small as 7x7 and as large as 20x30. It really depends on the image, the space and what you're trying to accomplish visually. Almost any size can be successful if it suits your image (composition, technique and subject matter), the space in which its being exhibited and the target audience/market.