So, let's assume I have taken some great shots, and secured a gallery presentation. I have shots of extremely varying aspect ratios, and many themes. Now I have to order prints. 

> What size should I print at?

I am assuming that the shots were taken at a high resolution, so the prints can get pretty damn big before they lose quality, but there is a huge range of sizes I could pick below the maximum. 

I know that this will depend on the shot, the space, the subject, the room lighting, and hell probably even the weather, but where to start? Is there some information written about how to make this decision, or can someone offer some rules of thumb or even suggestions?

This question seems overwhelmingly subjective, but I am hoping, somehow, that we can come up with some advice that is universally *useful* at least.