Hi Diana,
It looks like you can calculate average family size from Table B09019. I was able to match the national average (3.24) using this formula:
Numerator = Row 3 (Population in family households) - Row 18 (nonrelatives in family households) = 256,766,398
Denominator = Row 4 (Householders in family households) = 79,241,578
This is a bit clunky though- there may be a simpler way to do this.
-Mark