Woodcraft Magazine Vol 1 No. 1 (Jan 2005) showcased these banks.
The mailbox doors are 3 1/2" X 5"
I scanned the diagram image and you can find it here:
http://pages.suddenlink.net/boccie/Mail_Banks.jpg
It is 8MB so might take a minute to show.
Hope that helps.
If you need any information from the article let me know.