Just my 2cents worth, so take it for just another opinion. I would give extremely serious thought to exactly what you're going to hang. As a general rule, the strongest presentation will have a unifying theme through all the images, beyond 'my favorite 25 images from my inventory'. Since none of us are Richard Avedon or Ruth Bernhard, we're not in a position to have a life's retrospective that will justify 25 images on different topics. I think you would be far better served adhering to "less is more" and showing a single coherent, consistent body of work (consistent in size, subject matter, and/or presentation) than having a scattershot blasting of color, black-and-white, large, small, enlargement and contact print. You would be better off having 15 prints up that are all about a single topic, regardless of size, or all presented the same way even if they are of disparate subjects, than you will otherwise. You'll stand out from the crowd that way too. Far too often in group shows that I've participated in, I have seen people try to fill every square inch of wallspace they are alloted, only to have their good work drowned out in the cacophony on the wall. So my suggestion would be to edit, re-edit, and re-edit again before you hang the show. If you have the chance, go to the exhibit space beforehand, sketch out a plan of the space, and then try to envision how you would hang the work. This may help you with your editing process. Other than that, previous advice offered is good- aim for eye level, and a reasonable distance between images so that your viewer can take in each image without being distracted by the next, losing focus and ultimately forgetting the image.