Well if you aren't concerned about the challenge VTs can handle pretty big files. MapTool is limited to about 5000x5000 for the resolution on bounded maps.

For the map resolution think in even increments of what the VT uses as its base. For example, MapTool uses 50 ppi so 100 and 200 are nice increments that allow zooming and maintaining quality. Other VTs have different resolutions so you would have to check their specs. For the challenge though you have to worry about file size regardless of map size, but you do have a minimum size as well.