Make sure you select a seat which either sticks out into the exit row or is a bulkhead seat, that way your feet can poke out a little more. Usually the footrests fold down from the seat in front, but with lack of seat in front the footstool comes up out of the floor and goes flat with nothing to stop your feet hanging off the end.
By my judgements and recollection the following 747 seats should fall into this category
Of these anything on the upper deck is great, I always go for 62J/K or 64J/K with my wife, but as these are well known as better seats you may find they've already been snaffled by BA Golds/Silvers or those with pre-allocation rights. If you are able to pre-assign then go for this ASAP!
"My first job was selling doors, door to door, that's a tough job innit" - Bill Bailey