i believe there was a question asked SOME time back (a few months) about how to increase a map's size when it was at the WE max but not the WEU max, there MAY be some helpful information there.
i believe the WEU max (400x400, right) is tops (no experience with WEU however, so i'm just assuming), and that anything bigger would A) be a bitch to terrain B) be a pain to traverse C) be bigger than you would actually need for anything other than bragging rights or single-player campaigns.