Just to reiterate how different sizing is among manufacturers, my two MTBs are thus:

Titus Racer X size XS (unisex), size 15.5" (second smallest size they make)
Novara Bonita size S (WSD), size 12" (smallest size they make)

Both bikes have virtually identical standover and virtual top tube lengths and both fit me perfectly!

I am 5'2.5" with a 28.5" inseam.

Emily