Hi Karen,
I am not an organizer of the competition, but I am a judge. Jane Morgan, who is the main coordinator is on Jim Abernethy's Shearwater this week!
The intention of the rules is simply to ensure that prints come in at equal sizes - we don't want entries that gain advantage by being printed bigger. We don't get a ruler out and start measuring them!
A4 is a widely used paper size measuring 210 × 297mm or, to quote Bill, 8.27 x 11.69 inches. I realise that using this standard is a bit confusing for photogs from the countries that don't use A4 (particularly the US and Canada, where there happen to be rather a lot of underwater photographers). So sorry for that.
A4 is pretty similar to Letter - quote Wiki: "A4 paper is 6 mm narrower and 18 mm longer than the "Letter" paper size, 8½ × 11 inches (216 × 279 mm), commonly used in North America."
Anyway, as far as we judges are concerned it is up to you how you use that space. Most single print entries are usually printed filling most of the sheet - leaving a small white (unprinted) border around the sides. Some go right to the edge. Personally I think they look better with a small border, but we judge the prints - not the borders!
Printing a small image with, a large border is unlikely to show the details of the image well, and is unlikely to impress us.
A un-cropped (Nikon/Canon) DSLR image is 3:2 in aspect ratio, which on A4 means there will be a slightly large margin in the shorter dimension than the long one. So we see this quite commonly on entries - don't worry about this. On Letter paper this would be exaggerated further.
Have a read of the judges comments from previous years as a guide to selecting your entries - and good luck.
2008 - http://www.divemagazine.co.uk/news/article.asp?uan=4323
2007 - http://www.divemagazine.co.uk/news/article...amp;v=1&sp=
Best regards,
Alex