Your household size includes you, your spouse, and dependant children.  You will also need to add the total number of people you are sponsoring.  Don't forget to count yourself as well.

Sample:

you = 1
your spouse = 1
children = 2
parents being sponsored = 2

Total amount for the number of people you will need = 6

If you don't make enough, just have a joint sponsor fill I-846 in addition to yours.  Anyone can be a joint sponsor as long as they are a GC holder or US citizen.