There are no specific guidelines that a Little Free Library must follow. The only requirement the Little Free Library organization asks for being an officially sanctioned Little Free Library is that the builder fills out the form located at LittleFreeLibrary.org/ and places a charter sign and number on the library.
Builders may put the name of any other builder, sponsor or organization on the library. Registration and the installation of a charter sign allow the builder to photograph his library and display it as an official member of the Little Free Library network. The Little Free Library name is registered and trademarked, meaning that it is important to register the Library before using the name.
The Little Free Library organization does offer a few tips for building the library. Suggested dimensions are 20" wide by 15" deep by 18" high on a post or sturdy foundation, but those specifications can be altered to fit your needs. Little Free Library recommends using recycled or found materials and demonstrating green building techniques and materials. It also suggests using techniques that make the Library resistant to weather and deterioration. Further advice on how to deal with adverse weather and other environmental conditions are found at their website.