Well, Treir is described in the books as very tall. In her first appearance, she's described as being a head taller than the Orion male she's accompanying, and Orion males are pretty big as a rule. Halle Berry is only 5'6". Even if you wouldn't choose Suzie Plakson (who's over 6'1"), you should still choose a woman of more Amazonian proportions than Berry.
I just picture her taller
This isn't "real" casting, for me. I casted the young Winnie Cooper as Thriss.