SArah, get a copy of the competition rules. They will specify the max. size of mount, colour maybe and the min/max size of your print. For a competition, I doubt you would want to go much smaller than A4 unless you want a square format. When you are starting off, it is perhaps easier to get some mounts cut with a (slightly smaller than)A4 size aperture and just print your pics at A4 size. I found it a lot cheaper to send away for precut mounts than to get them done locally (I was quoted £7.50 per mount locally for a 16in x 20in white mount with < A4 aperture!!). Or you could buy a sheet of mount board, cut it to the correct size yourself (no aperture) and then just stick the photo on it with double sided tape. That may give you enough time to get organised for the next competition. Once you start messing about with different sized prints and non-standard sized apertures, it is probably time to get your own mount cutter! Good luck!