Airhawk's own web site gives size 'small' for the K1200S, which I believe is the same seat as the K1300S.
However I think I would overlap that size despite being of reasonable build.
I know the medium has been touted on here, but has anyone compared the two sizes for suitability on a K1300S standard seat?
Or does anyone know of a stockist in easy reach of Swindon rather than mail order?