THe first question that comes to mind; what does the gallery want?
WHere are these going to be hanging (how much room do you have)?
Are other images than yours being hung as well?
What is the layout of the gallery?
All the above may dictate the final decision.
The rule of thumb is generally ( and this is a very global statement, so everyone; don't crazy) to hang all images in the same frame usually "salon" style.
This does not mean the images are all the same size, rather the frame is hung vertically and the same size;; usually 16 x 20.
However, with all that said, we are working on a show for next year and as the gallery is an old house, different projects will hang in different rooms with different frames and sizes. Each project has a specific overall frame size and hanging direction. (hope that makes sense). Basically, 80 % will be framed with black metal, the rest in contrast grey or antique wood frames.