Thanks for reminding me to return to this topic! I wound up wimping out and borrowing smaller rolling luggage that did fit overhead. But then I didn't see anybody asked to check the size or weight of their bags at IAD, and other people seemed to be taking advantage of that skycheck service without being asked to produce a credit card. So my only regret about my booking wound up being not paying a little extra to reserve a seat that would let me enjoy the view of Toronto from the right side of the plane.

So last year, I had my usual rolling luggage that fits just fine above a seat on a 737 or A320 but not at all on an E145. But the flight was maybe half full, so instead of giving up after boarding and having the bag gate-checked, I stashed it on the floor under the vacant seat to my right, where only part of it fit under the next row of seats and the rest stuck into the legroom of that fortuitously unoccupied seat. In retrospect, there's no way that was kosher for evacuating the plane, but nobody said anything about it at the time.