Nesting box/Sleeping area Question

This year I decided to add a few hens to my zoo. I started with three, 2 ended up being roosters. Sent the two back, got two more, started getting eggs, thought this was just dandy and last week I added 3 more 3 month old pullets bringing the total chicken population to 6. The older ones (May/June hatch) are being good girls and roosting where they are suppose to. The new pullets have decided that squishing into the nest box is a fine place to sleep. Needless to say they are making a mess in there and my previous spotless eggs are now messy too

So - any suggestions on how I can convince the new girls where they are suppose to sleep? I only have one nest box since it was enough for the three girls. I have plans to add another but figured I had a couple of months since the new ones won't be laying for a bit. I'll move those plans up but things are busy with xmas so I'm not sure when I can knock one together.

For the new nest box, the FAQ says 12" square is good for large breeds - any comments on that?

Pictures of the girls and their coop-mates can be seen at:
http://pets.webshots.com/album/560480458vxbPKF

Thanks!

Donna
1 Australorp, 1 Americana X(feathered legs), 1 New Hamspire Red X, 1 Barred Rock, 2 RIR, 4 Call Ducks, 8 Pigeons, 1 Golden Retriever, 4 Nova Scotia Duck Tolling Retrieves, 1 Mindy