First, congratulations. Not everyone has such an opportunity. Second, what do you shoot? Landscape, nature, figure, street, still life, abstract? You should be able to put together images that go together or contrast, depending on your style and taste. Take out your work prints and tack them to a large wall, or on the floor. Take a good look and start gathering the ones that appeal to you, see if there is some cohesiveness to the group. I think it was Stephen Shore who said, only put as many prints in a book (in this case, a show) as your age. As far as size goes, there are a number of factors: the space you will be showing in, the sizes you can print in your darkroom, how the prints look best--some are more intimate and work at 8x10 or even smaller, some need to be 16x20 or larger. And how much money you can or are willing to invest. How do you and the gallery want to present the work? Pinned to the wall, just matted? Framed?