As far as i know you can't.
You can color or use symbols with different sizes but you can't change polygon areas.
Yes, it can be done--you will build a cartogram. I've never personally done it, but saw this article recently: Cartograms in Tableau - Clearly and Simply. Refer to the heading "Collecting and Preprocessing the Data" I'd like to try it one day, but I don't have a specific use, and since it requires processing outside of Tableau, I haven't pursued it any further.