Rich,
In answer to your question, the shorter you are the better. As with many vehicles of this type pedals and seating can be adapted to a certain extent but the 917 cockpit is small and sparse and ideally you want to be under 6ft. You lay down in it, rather than sitting and the roof curvature to the side is very tight if you are wearing a helmet.
To comply with regulations Porsche did have a passenger seat in place but it was only two thirds of the size of the drivers one, and once again you could adapt and put a full size seat in, but leg room will be EXTREMELY tight.
I,m 5ft10inches tall which is great for this type of car and the attached picture gives some idea of the driving space. You do not have any room above your legs as the steering rack and anti roll bar are right above them because of the very forward driving position.
Hope this information helps,
Graham.