Most of the arm style senders are driven by the depth of the tank. The arm is then adjusted/cut to fit the tank. There should be a mfr and/or part number stamped on the flange. Try lightly sanding that rust to see if it reveals the info.
As far as other options, you can also replace the arm style with the reed type sender, which is a straight rod with a float. Again, tank depth would be your selection criteria.