ProTip: you can also fit a 22x14x9 rollaboard underneath the seat in front of you on the C/D side, if you are desperate.
And by "desperate" I mean the time I volunteered for VDB but they ended up not needing me, and the GA (frantically trying to get the flight out on time) rushed both me and #1 off the standby list down the jetway and practically shoved us onto the plane. The FA saw my bag and said "we might need to check that" and the GA said "no time -- does it fit underneath?" Me: "Uh, I've never tried." It *almost* fit all the way underneath the seat and no one seemed to care