Can someone help me with brooder styles?

[kressg23]

Hi All,
I need to get my 7-week-old RIR's out in the coop with my 11-week old bantys. The RIRs are inside my house and are starting to get pretty smelly, so time's up.

Anyway, I think the only option to introduce them successfully is to build a brooder of some sort and put the younger chickens in it for a while, till the others stop being so aggressive. I'm trying to find design ideas. I've got a fair amount of room to work with, as our coop is pretty good size. I'm thinking about something maybe 3x5? Is that big enough? My concern is not so much for when they are small, but when they are this big. (We will be getting more baby chicks at the end of the month.)