[amaks]

"But once you start to get bigger, say the 15 person range, it starts becoming unwieldy."

I think this is a good indication that a team is getting too large, needs to be split into smaller teams. There is a notion of "two pizza teams" described by Werner Vogels (http://derivadow.com/2007/02/20/two-pizza-teams/) to keep things efficient and organized. For teams this small, open floor is very productive and efficient for collaboration, including software development.