I have been working out what size photographs to get printed for my exhibition, I worked out that if I have a 5inch gap between each photo, I can have them printed up to 24inches (height).
After looking at printing prices, I've found that this website seems to be the cheapest:
http://www.photobox.co.uk/shop/wall-decor/poster-prints

I'm thinking of printing mine in either 16x12 or 18x12, as I think this is more affordable and the size is fine too as I need to print 4 photographs.