Not to mention that the main purpose of the challenges is that it's a friendly arena for people to produce a map to a deadline so that they have an incentive to finish it. You will see that in the WIP threads people taking part in challenges help and offer advice to others, which is how we want it to be.

Because the challenge is really there for people to improve their own mapping skills, I guess we don't like to put too many restrictions on sizes / formats etc which would (IMHO) needlessly make the exercise more complicated for participants and might put them off from taking part.

Bear in mind also that not everyone has the same sized screen as well.