It must go in the overhead locker, and the cabin crew are normally very careful to check and enforce this, even if it is a small handbag. The bag size must still conform to the second bag dimensions, not least because you may be shifted from that row. Also on some aircraft it would actually be row 10 that is the exit row. Basically if your aircraft has not got row 13 you are unlucky, the seating is more compact and the exit row is not row 9 but row 10. You won't know that for sure until you get onboard, but at 3 days from departure you will get a very good idea.