How do motorcycle companies determine the seating position of a motorcycle rider?

That really depends upon the bike. The seating position is usually one of the major factors they take into consideration when deciding what type of bike it is to be. A tourer is designed for an average sized person to sit comfortably for long hours, a cruiser for them to sit feet forward in the classic style, and a sports bike so they can get down at speed and shift around for scratching. On trail bikes they pay little consideration to seating and more to how the rider balances their weight on the pegs and the relevant COG (centre of gravity). Obviously they compromise somewhat for costs and practical considerations but rider position remains a major factor in any new bike design. BMW have probably done the most work of any motorcycle manufacturer in their seating, with their ergonomically adjustable design work and crash testing. Doc