We debated on a specific size, but determined against it. The main reason (though we have had some specific challenges in the past that did otherwise for good reason) is that people have vastly different hardware capabilities. Ascension for example starts most of his maps at 3000x3000 or larger. However some people just can't do that with their older hardware or low amount of RAM in a raster application such as GIMP or PS. So, generally, unless the challenge specifically says make it this exact size (which will be rare), then feel free to make the image at whatever resolution and image depth you need to get you vision on to paper(so to speak.)

If you want to see one that had a specified limit, check out the challenge we did in (I think) August 2008 which was to make dungeon times to be printed. The idea was to be able to print to standard paper, trim the white edges off (or overlap) and assemble multiple tiles to make a dungeon on the fly. We have also had one where we created map for a VTT use and you could make it any size you wanted as long as it was larger than X AND the filesize was below Y.