anyone know the size of a standard mail box that would have been fixed to a house in the late 50's or 60's ?

How exact do you need?

Go to ebay, and search for "vintage mailbox" Find one like what you are looking for, and see if the seller posted any dimensions.

thanks Kevin will try that need size to draw plans of a building off a photo.
jim

I would suggest doing a Google search for USPS mailbox size.

Mailbox sizes are regulated by the Post Office, and have been for some time.