A nominal 20" bag would probably meet the requirements with wheels. But for the most part the airlines haven't really been enforcing the smaller size limits unless it doesn't actually fit in their bins.
http://www.nytimes.com/2015/06/16/upshot/your-giant-carry-on-bag-is-safe-for-now.html?_r=0
It’s not surprising airlines would be reluctant to introduce a rule to promote smaller carry-ons, given the way they treat the rules they already have in place.
“In the past, U.S. airlines have not enforced their existing carry-on luggage standards, so American consumers continued to purchase the larger sized carry-ons,” said Stephanie Goldman, a senior director for marketing at the luggage manufacturer Samsonite.
She’s right. Go get a tape measure and check the dimensions of your rollaboard suitcase. You might find, as I did, that it’s 15 inches wide — an inch too wide to go on a plane, even though I’ve actually taken it on board hundreds of times across many different airlines. United even
sells a “carry-on” suitcase through its MileagePlus store that is, technically, an inch too deep and 2.25 inches too wide to carry on a United plane.
Airlines have reasons not to be sticklers about carry-on baggage. Tight enforcement means testy, time-consuming exchanges with staff as passengers squeeze and shove to show that, if you try hard enough, their suitcases
do fit into a size-wise bin. Additionally, by letting their most frequent fliers board first, airlines ensure their most valuable customers get access to bin space, even on flights where there isn’t enough room for everyone’s bags.
