Got 2 Canyons Nerves 2015 for me and GF this year, and it is a big worry not being able to try one out beforehand.
I’m 5’8″ and was looking at Canyon Nerve 29er, played with the PPS system ( it is only interested in you leg measurement ) and managed to get it to say medium by retaking measurements until I got one that worked out :-).
Glad I did, the seat is about 3″ out of the frame with the dropper fully extended, and the reach is perfect ( they are a short reach as everyone says ).
Going through the figures for the GF ( 5’6″ looking at Nerve 650b Womens Spec ) and she came up small and is more than very happy with her bike. The saddle is all the way back though with the inline dropper post.
When she 1st got on mine she was worried that she should have gone medium, but thinks she made the correct choice now.
So to me it sounds like your GF is right on the crossover, sorry to be of no help whatsoever :-).
Not a Spectral, but if your at Mayhem this weekend your welcome to have a go.
P.