How much larger should a bga pad be than the ball itself?
Intel's BGA packaging guide has some good guidelines:

http://www.intel.com/content/dam/www/public/us/en/documents/packaging-databooks/packaging-chapter-14-databook.pdf

Check out pages 17 through 20.

They recommend a 20 mil pad with a 24 mil mask opening, for 2mil annular ring.