Giulianna, that's so odd, because the women's medium MTBs from Giant are listed as being 20inch.
OP, I'm taller than you and am somewhere between sizes. The one shop I went to, the guy hesitated to try me on anything smaller than the 17.5 inch because he thought I'd feel cramped. I'd probably be on a small unisex 29er, maybe a medium WSD depending on how the sizing goes.