I think it depends on what you need it for. If you want it for a carryall or a work bag I'd go with the medium. For an everyday bag I think the petite is a good size. I mean, it's not really that small. I'd consider the petite, especially at your height unless you need the extra room for a specific purpose.
Thanks uadjit, you are making me leaning against petite

I dont think i need that big a bag anyway,and the petite is looks very "well proportioned".