It shouldn't take up that much space. You are over complicating things. The biggest file would be the character itself, which would not take up as much space as you think. Think of all the VC games you can stick on your Wii, how many models do they include? How many animations? A ton, such as mario 64. And mario 64 doesn't take up all your Wii's space, does it?
For dlc for Brawl, all you need is one measly 3d character. All the stuff is set up in the game already. If they were to add dlc, then would have prepared for it on the game disc itself to make adding in new characters less of a pain.
But we must wait til this Friday for Nintendo's speech and hopefully they will say something about it.