That way exceeds the dimensions for BA's personal item at 50 x 32 x 22, plus exceeds the free bag on Ryanair. Granted, you'll get away with it on BA as they pay zero attention to personal item sizes but unless you're 4 foot tall, where do your feet go if that's maxed out and under the seat? On the basis it's the personal item and should be under the seat, not in the overhead lockers of course...