Quite honestly, I can't make up my own dreamcast for any book characters, and I usually just go with the flow. So, I'm going to have to go with Saorise Ronan. Yay for not being original! ;)
Saorise Ronan has that fierceness that Tris has, which I guess you could see in her movie Hanna. She also seems pretty small enough, like how Tris is normally described in the books.
*Picture from imdb.com