If you can skip the wheels and use something like Red Oxx Air Boss or Briggs Riley 235X, then the only advice is not to overpack it deliberately.

However, with wheels, I am afraid 20" is the maximum and I would even feel more comfortable suggesting 18".

Bags on wheels, with a 40" handle extended, just look bulkier than a bag hanging on your shoulder, and are also more likely to attract attention.