The Baby Surprise can't be pinpoint size precision because of it's construction so a lot of it is experience and luck.
This page gives you the approximate yarn and needle size for various sizes. Scroll up just a bit to see the chart.
It seems to corroborate what I said..worsted weight and size 8 needles. Smaller needles make it slightly smaller, larger make it slightly larger. Depends on the fabric you like, too.