As Ms. Grace says, it varies, and just like with cis girls, you get what you get.
My areolae are about 3X the size they were when I started 5 years ago, and darkened a bit, from brownish pink to light brown. I've heard that's more or less normal. (Of course, not all cis women who get pregnant have permanent color change either, and not all cis women *need* to get pregnant to have the color change at all.) It's also variable what size they'll be relative to the size of the breast - my wife has huge breasts, so her areolae are proportionally much smaller than mine, for example. Some women have big breasts and small nipples/areolae, some small breasts with large areolae, etc. But HRT should do *something* for you in that regard.