Well, the space-squad idea could work, if we be careful to not make the 9001th starwars(c)/space marines(tm) fan-game, like said before.
I don't know if that idea has had any progress yet, or if you're planning to go for another idea by now. Maybe I could get (read-)access that doc you're all talking about ?

About the size of sprites/tiles, I'd say keep it small. It's supposed to be a squad game, so you'll need to see your squad (AI/other players). The size Jazz2 uses might be a good size to start with. And you could always use 3d-functions for drawing the 2d-graphics, those make scaling your viewport easy so the scale could depend on the location in the lvl or the amount of players/action onscreen. (Although GMaker hates 3d)