Used the medium for a 13lb mini schnoodle on Delta over Christmas.  This case is the exact size requirement Delta gave us for use in a regional jet and it fit snuggly under the seat in front of me.  The problem came when we boarded a 757.  It only fits under a window seat (which I didn't have) because of all electronics under the middle seat and the aisle seat storage is too narrow. It did protrude, but the flight attendants did not say anything. Great case though.  Easy to roll.