Didn't they say the gametype was an "invasion" type??? Meaning you go in, complete your objective, and get out? If thats the case then I am sure the defending team has to hold off the offense for a certain amount of time before the game ends.
But either way, from screenshots in magazines and online, it seems that map size will vary from corridor combat to huge open levels. I for one enjoy maps like Blood Gulch and Sidewinder, but can't stand maps like Prisoner or even Hang 'Em High. But then again, I have friends that only wanna play those 2 maps.
Either way, to all his own, but I'm glad that Bungie is introducing many different game types, and a variety of map size in their next classic.