I made mine 16x16 feet in order to have walls and roof at reasonable height and still be able to see over them to a reasonably low angle with a Dob. It also leaves room for a 2x4 foot table and a couple of chairs and shelves. IMO you will wish you had made it larger. It's not too late to enlarge at this point. All our local lumber yards carry 16 foot lumber, a couple even have 24 footers. Try another lumber dealer.
-------------------- ?
Observing since 1966
Messier Cert #898