In order to be safely stored in the overhead bin of an airplane, any carry on bag can be no more than 22” x 14” x 9”. However, these measurements can vary a little from airline to airline, so check with the airline you’re traveling with to be sure. Even so, the point of taking a carry on duffel with you, as opposed to a structured suitcase or wheeled bag, is that the unstructured aspect here can be quite forgiving. So you can probably get away with a slightly bigger duffle bag as long as you don’t overpack it. Bags that are mostly stuffed with clothes can probably squeeze into most overhead compartments.