The Post Office requires mailboxes to be between 42" to 48" off the ground.  The Department of Public Works recommend that your mailbox be 6" back from the edge of the road.  In this diagram, the mailbox is 18" long and the post is 24" from the road.  Obviously, different mailbox designs will require a different set back of the post, but, if you keep the mailbox 6" back from the edge of the road, then the snow plows should pass freely.