All magazines I could find in stores such as WHSmith and Supermarkets were A4 in size. This is the standard size used and makes it easier to manufacture and also to fit in the slots in shops where the magazines sit. Recently fashion magazines such as Elle and Cosmopolitan have produced a 'handbag' sized issue that is A5 in size. This appeals to the 'on the go' women who will take their magazine with them whilst traveling.
Creative Review is a well known exception to the standard A4 size. It is square to stand out from the crowd. I find the size quite hard to hold whilst reading and also I found that it could also get lost amongst the other taller magazines. Because of its stunted height it is not as prominent on the shelf.