Much to the amusement of my husband I have just measured my 'seat' whilst seated, also my shoulders.
Seat is 14.5"...so no problem there, but shoulders come in at a tad over 17".
I never used to have a problem with economy seats (I'm not that tall and my legs are not that long) but I can see the shoulder room being a problem.
I really don't think I need much of an excuse not to travel down the back if I can afford better !!