I think 6-8 is a good size expansion pack. BF2:SF had 8 and two became dominate. Though Karkand was dominate, there were actually 4 or 5 maps you could usually find all the time.
If they do make 1943 an expansion (which I think they should btw) with 4 maps, new weapon loadouts, new vehicle skins, and the awards, it wouldnt be that bad of an expansion pack for $10 to $15. They were going to charge $15 for it anyway for it to be on the exact same engine and setup BC2 already is at.
What would be funny if 1943 expansion pack is on our harddrives right now and we just need to unlock it I bet people would rage quite.