The Cornell Lab of Ornithology is my go-to source when it comes to all questions about birds, and they have more than a dozen plans on their site along with a complete list of entrance hole sizes:
Thanks so much for your reply! This proves to be very helpful! Bring on the Spring!
Birds and Bloom e magazine, here is the url : http://www.reimanpub.com/shared/pages/onlineemail.aspx?codelocal=Newsletters/BNB/2012/08/newsletter.html&firstname=R.&firstname.lastname@example.org&refurl=IMHDB02V&_mid=2377906&_rid=2377906.970336.120737
follow link for project of the month and you will find a link for all kinds of houses, feeders and other ideas.