I decided to print off all the posters to submit in A3, I also printed them off as A4 for myself to keep which I will mount on foam boards. I changed the image size on the psd file, saved it as a jpeg and took them down to the Nottingham Trent University print shop to have them printed off.
Below is the link to the price list, they are very reasonably priced considering my previous blog post on printing at the beginning of the project.
Nottingham Trent University, 2016. Information Systems [online]. Available at: https://www.ntu.ac.uk/information_systems/document_uploads/85125.PDF. [Accessed 21 March 2016].