Hi!
This post from a while back should do the trick, though involves using javascript. It’s almost your exact situation, as it turns out. Also, it’s in Axure 8.
By the way, the reason it doesn’t work using the size of the widget in Axure is that Axure doesn’t grow the widget to accommodate text: instead, it just lets the text overflow the bounds of the widget without changing its size,.
This script accesses the text span inside of the widget (which actually contains the text) and gets its size.,