I like (and upvoted) most of the answers here. But there is one further point that really needs to be considered. And that is inventory management. Gallery represented artists must deal with this or you both crazy and/or broke. Yeah, not print quality, no esthetic rule of thirds guidelines, nothing about metal or wood. But inventory.

Face it, prints are relatively cheap compared to frames, mattes, and glass. I'm seeing about a 5:1 ratio between my frame supplies cost and my printing costs. With this is mind, you need to realize you can afford to print photos that won't sell but you can't afford to buy frames for photos that won't sell.

And this means you must be able to re-use your frames and mattes.

The easiest way to do that is to restrict yourself to a small number of sizes. Personally, I only sell 11x14 and 10x20 prints. I have stocked materials for 12x12" prints but frankly, I don't shoot much in that size and the ones that I have haven't sold.

And yes, this does affect your craft. Sometimes a shot just can't be cropped to either of those sizes. But you know what, it probably can.