How big is an average/ larger movie theatre screen?

Answer 1

Good question.
The Somerville Theatre, near Boston, has several screens with sizes 18' x 12', 25' x 15', and 30’ X 14’. The main theater has the 30’ X 14’ screen, so I think 30’ X 14’ may be the answer you seek. (Source 1)

At Lincoln Center, NYC, the Walter Reade Theater boasts:
"Screen Size: 35' wide by 19' high. Video Image Size: Approximately 12' wide by 9' high to full screen." (Source 2)