I will clarify what I mean by "arm".
I am assuming you are not referring to the actual wiper arm (that tapers to a "hook" at the tip) that is permanently attached to the motor assembly.
I am referring to the wiper blade "arm" that contains the blade. It is sold as a complete assembly, so the blade and arm are the same 22" length. It even has a plastic "wing" that assists with keeping the blade firmly against the windshield during those windy torrential rainstorms. Don't fret...the wing is subtle, not like those tacky neon wiper arms we used to see in the 80's.
All my MBs have the Bosch arm and blade assemblies, and I have been happy with the results.
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car