To "paraphrase" Gui:
If you have a nice idea for a map and it just so happens to be a 3000+ territory idea, sure, go ahead and make it. Put please, don't put 3000+ territories in a map just because you want the "honour" of having made the biggest map.
Please keep in mind that Fizzer has to manually approve all maps before they become visible to other people. There are some [published requirements](
http://wiki.warlight.net/index.php/Map_requirements), which your map will definitely be checked against. I wouldn't be surprised if there's also some "unspoken rules" which will be taken into account though; if you make a 50*60 checkerboard map, it just might be rejected (and your effort wasted) simply because it's incredibly lame. (Contact Fizzer if you need to know how it actually works in detail; I'm just guestimating here.)