I suggest you measure yourself across the hips and see what the size is.
I have a 42" waist and I find that a 17" seat is a touch too small. I would also counsel you to watch out for seats promising more legroom. In some cases because of their positioning the tables are located in the armrest making the seat just that bit smaller, so you sacrifice one dimension for another. As most of these are beside the exits on some a/c you may also encounter a problem if you book one, as CAA rules require those sitting there to be mobile and able to assist in an emergency and it could be judged that you could have problems in this area when you board and be asked to move.
I would suggest that without hesitation you will need a seat belt extender because these seem only to fit waists up to about 42" or maybe 44" at a push.
Remember ultimately that it is a long way if you are uncomfortable in your seat so I would certainly speak to BA at an early stage.