My wife owns the Spectral:On WMN 6.0 in size XS and she ist 1.60m (about 5'25''). For her it is the perfect size and the seatpost is in the lowest position. I am 1.80m (abaou 5'84'') and size M is on my Spectral:On 8.0 is perfect for me. Maybe you can also try size S.
We are using our bikes on...