Household size (HHSIZE) is a function of the portion of the population that is of pre-work-force-entry age (POPREWORK); the bigger that population that has not begun to work is as portion of the population, the larger is household size.  

Internal to the model the denominator of this equation is referred to as the household intensity, which falls as the pre-work age term rises.  Thus the household size rises with the pre-work age term.

There is an additive shift factor calculated in the first year of the model run to assure a match of the calculated and empirical values; that shift factor decays to zero over 100 years.