Agree with Nathan... Baby boxies don't really have averages, although min and max are probably a good way to go.
Just to give you an idea, my babies from last year (Aug 02) range from 20 - 30 grams. From the year before, (Jul 01) 40 - 105 grams. Diverse huh?
Don't obsess too much about sizing... Truth is, they grow very slow in the wild and don't eat nearly the volume we feed them. If they look healthy, show energy & marked smooth growth in both size and weight (< a better indicator), you have little to worry about.