I found that certain versions of the Heroclix Giganta figure (giant Human female in a leopard-skin bikini) is a size match to my old, 1990-vintage, Ral Partha Storm Giant figure. So I ordered three different versions of Giganta, from e-Bay, and one of them is, indeed, a near-perfect size match!
I also bought what I believe is a Paizo Storm Giant figure. He is pretty close in size, but his skin was not purple. I used a purple which is a poor match to the Ral Patha figure which I had pro-painted by a friend, circa 1993, but they work for me.
Take a look at the three figures, side-by-side: Ral Partha, Giganta, and Paizo.
I think Giganta would look superb in an ankle-length gown. I will need to paint her bikini a muted color so it won't show through. Then I can take an old, thin, cotton handkerchief, soak it in 50:50 PVA White Glue and Water, and drape it over the figure appropriately. Once it hardens, I can paint it. I think a long gown would be more appropriate for a Storm Giantess, than a leopard-skin bikini. Here is a comparison of Giganta-as-a-Storm-Giantess, next to a Paizo Cloud Giantess. I think I can add a nice gown, and jazz it up a bit with some gold and silver paint embellishments.
I picked up three different Giganta Heroclix figures, all different sizes. One of them should work as a female Stone Giant, matching in height to my other Stone Giant figures. I'll paint her skin gray, of course. I believe the figure's hair is a mid-brown color, from the factory, but I will likely paint that white, or a shade of gray, as well. The leopard-skin bikini should be appropriate for a Stone Giantess, however. Cheers!