It's been ages since flew IB in Y, but I think they introduced a new "Comfort" Y fare that includes an XL seat.

I'm surprised that a OWE couldn't choose the seat - so maybe your status isn't being picked up properly - but assuming that all is well, you probably won't be able to request an XL seat at the airport. (the XL seats would be randomly assigned when the flight is very full and blocked off for payment if not)

Onboard, some flight attendants let you change seats. Others insist that you stay in your assigned seat.