Many manufacturers claim they make unisex boots that fit both men and women. Well, a claim is one thing, reality another. And the reality is that most boot makers say their boots fit both men and women because they didn't want to bother to make women's boots as well as men's. So for decades most women have tromped around the woods with ill-fitting boots designed for men's feet. That's because women's feet are usually shaped differently from men's, they're not simply smaller.
Women typically have a narrower heel, and an arch pattern that is slightly different.
Not that women can't fit into a man's boot, it's just that they often won't find them quite right. Surprisingly few boot makers build boots on women's lasts. Of course, finding a boot designed for a woman still won't ensure a great fit, as all feet are different and each boot maker uses its own last. So try several. And try some men's boots too-it's possible they'll fit best.