From the horses mouth:


Sizing
Mens 51cm fits most riders from 5' to 5'9 / Mens 56cm fits most riders from 5'9 to 6'2"
Ladies 46cm fits most ladies from 4'10" to 5'7" / Ladies 49cm fits most from 5'7" to 5'10"
If you are between sizes, choose the smaller if one has shorter torso/arms or the larger for longer arms/torsos.

Looks like a ladies 49 would be the best fit. Add truck nuts if you need reassurance. Nice looking bike.