I head that SC2 will features online campaigns with maps that are linked, soes this mean that you can make many differents maps (like for an RPG for example) that works online if each map don't exceed the size limit allowed in online map
Yes, they do plan on adding this feature into the game but I believe they mentioned in their Q&A at the Blizzcon '09 panels that they currently don't have plans on including this feature with the game release.
You can, however, loop the borders of your maps. This means you can make a unit leave the middle-east boundary of the map and reappear at the middle-west boundary, giving a very large 'epic' feeling. As for the size of the map, they've only mention it's X by Zs, and there's no real way to determine that size.
Ghan has said he has fixed this. Monovertex please confirm this fix. This was only a problem with people that had signatures in the upper levels like not the special members but the respected members.