Where can I get plans to build a bird house? What are the correct dimensions for each species?

Birds that build nests in tree cavities are called "cavity-nesters."  These include bluebirds, chickadees, some owls, woodpeckers, kestrels, and many others.  Each species has its own "ideal" specifications that it looks for when choosing a cavity for nesting; so when you provide that cavity in the form of a nest box, you are more likely to attract the desired species if the box meets the bird's preferred design.  A classic book of 28 different nest box plans was developed by Carrol Henderson of the Minnesota DNR that is still available for purchase:  www.dnr.state.mn.us/publications/books/index.html.  Many state natural resource agencies have reprinted this book or have similar books or fact sheets.  Check out this link for a chart of design specifications for the birds most likely to use a nest box.