This is an area I collect.
There are wide variations in the size of "cut squares," but I collect "full corner" cut squares. These are generally 2" x 2" unless the design is greater than 2" as occurs with some of the later issues.
A full corner cut square includes all of the side and backing of the issue. Here are a couple of examples:
Since what you have is not a full corner, it would be best to square it off. Using the top and right margins as the square component, using a mount cutter, I would square the stamp off, leaving as much envelope as possible.