Apparently, multiplayer multi map support is implimented although it does require everyone to change map at the same time. Yet to see someone use this though (as those who would want to would generally spend time actually making 2 or more decent maps).
RTS games (the core of SC2) are not made for massivly enourmous maps and even 400*400 maps in WC3 caused lag on the best of computers if made well. If you want a huge map, reduce scale or break it appart into separate areas (each being a separate game).
Map size isn't what caused lag in WC3, what was on it did. I remember playing some 400x400 WW2 game, THAT was laggy. But on the flip-side, if I played a 400x400 galactic conquest, it ran fine. Simply because, while it was the same size, it had significantly less content. It was all just blank open space.
But ultimately, it should be up to the map makers, not Blizzard. This hand-holding, limitations they've imposed is the sole reason I wont buy SC2. It simply isn't their place to tell me how large my map can be, or what I put into it, or what I name it. SC and WC3 worked because people could litterally create anything they damn-well pleased, that doesn't exist in SC2. It's ironic because SC2's editor allows us to create almost anything, but blizzard wont.
If I wannt spend my time making a giant 400x400 map, that's a gigabyte in size, and that no-one will ever play because it lags. That's MY choice, Blizzard shouldn't be making it for me. If the map's too large, and lags. Surprise, people wont play it. Seriously, I can't stand this big-brother mentality, it absolutely ruins the game and it's such a shame.
My only hope is that we get a third party editor, like in WC3 and SC, and can truly create anything we want. Sure Blizzard will continue being assholes, and wont let us play our maps with other people. But atleast we'd be able to create them freely. :|