There are two types of exterior mobile home doors.  These are "House Type", and "Outsing".  House Type units have a storm door and the main door swings in.  Outswing doors, of course swing out.  The following will help you measure your door correctly.
House Type:
To get the proper size, from the inside of your home, close your door and measure to the outside edge of the frame that the door is hinged too.  Measure both width and height.  You may have to remove the trim around the door to see where the proper edge is.  If you come up with a measurement of say 33 ½” x 75 ¾” then you will need the door size 34 x 76.

Outswing Type:
Go to the outside of the door, and measure just the door (do not measure the frame).  The measurement you should come up with will be very close to an even number.