What are the specs for a custom Twitter background?

I am so frustrated!!! How do I creat a custom twitter background? I can not find the specs??? I have searched everywhere and can not find the specifications and requirements for creating a custom background on my twitter, yet I see people or companies using them on many twitter pages. Surely Twitter has the directions or specifications posted somewhere. I see where to upload the new background but not how Twitter requires the file to be composed ie: size, etc.

Please help me out someone?? It's annoyinh not being able to find anything telling me the information I need.