^
Really, the exact format doesn't matter TOO much, as I can resize. : p
But it's A4 210 × 297 mm. If you do it in Photoshop, you can go to Image/Canvas Size and select 'cm' or 'mm' and work with that. If you do it in Word, the size will always be right.
@majira: I think we should wait for relmy, if he wants to organise the 'contest'. ^^