Thanks HawaiiO! The camera rollaboard I use comes in two sizes; international (35.6 x 53.3 x 20.3 cm) or regular (35.6 x 55.9 x 22.9 cm). But from your comments, it won't be a problem. I was a little concerned about the economy segment.

For the layover, I think I'm going to take AREX into town, then subway to the palace & Bukchon area. Drop carryon bags at a locker outside security.