Worsted cotton on size 6s is going to be dense and heavy because you'll use more yarn. Use a lot larger needle especially if you're going to wear it over other shirts, a 10 or 10½ wouldn't be too loose.
The size of the item will depend on how many sts you cast on. With a 29" needle you need at least as many sts that will measure 29" or larger. You can knit up to 50" around on that size needle, just nothing smaller. Figure how many sts per inch you get, then multiply by the finished size you want. If your bust is 33", then cast on 33" worth; if it's 36" then you need 36" worth of stitches.