I want to install an electrical subpanel in a 2nd floor room, but am a
bit confused as to the location requirements. I would ideally like to
place the panel behind a door (e.g. a door opens into the room and
when open would be in front of the best panel location). This would
put it about 1.5' from that door when closed, with many feet of space
in front and to the left.

Also, is there a minimum height requirement? I would seek to put it
around eye level, but there are some roof joists tied into the wall
studs at about 4.5 or 5' and I think I'll have to go lower. Any
reason I can't have the top of my panel at about 4' (other than it's a
bit unusual)?