There's a rav group for it -
http://www.ravelry.com/groups/baby-surprise-jacket - and they have a wiki that includes sizing ranges, what yarn and needles to use for each.
http://www.ravelry.com/patterns/libr...se-jacket/wiki
Also, you may not know, but babies rarely fit the size of the `age' at that age. I'm making my newborn GD 6 mo size sweaters because she's large. They'll fit her when she's about 3mo. So a 6 month old baby will probably be able to wear a 9-12 mo size. It won't hurt if they're a little large, they keep growing....