All Focus stock speakers (I've read on here, so don't quote me) are 5x7. 6x8s will work also. You'll need adapters to hook up aftermarket speakers. I bought Ford OEM premium speakers (that $30 speaker deal that was a craze a while back), so I didn't have to get adapters. Same plug-in type. If you can, I suggest trying to go back in the threads here on Audio and finding the thread called "Upgrade Your Speakers...CHEAP!" There's a link on there to the great speaker deal on PartsExpress.com. It's a clearance sale, offering the Ford OEM premium speakers for $30 shipped for a set of four. That's what I got, and the sound is great! I'm selling my four stock ones, btw. PM me if anyone is interested! Hope all my info I have to offer helps!
2007 Pitch Black ZX3 SE with FocusSport CAI, custom glasspack exhaust, and 12" MTX sub in Subthump stealth enclosure with 540W MTX RT602 amp.