I still haven’t demoed one yet but I decided on a medium after having a quick go on an 18″ Boardman Pro 29er today. It had a 610mm ETT (6mm more than the medium Solaris) with a 70mm stem and 700mm flat bars. The length was spot on (could probably run a shorter stem and be comfortable too) but at max pedal height I had roughly 5 inches of seatpost showing, so a 19″ would be out of the question.