Hi Nathan,
That would be the Form I-134 for K-1.
The household size should be your household, any other dependents, plus one for the visa applicant. If you have any other affidavits of support on file for other aliens not in your household, you would also include those in your household size.
If the visa is approved and you eventually consider filing as a joint sponsor for Adjustment of Status, you would need to file a Form I-864 which has more complicated rules for determining household size, higher income requirements, and can possibly have much more serious legal consequences for you down the road.
If this is for a visa interview in Manila, be advised that they often reject joint sponsors outright for fiancée visas. The petitioner should at least be close to the required income levels and have the capacity, or clear potential capacity, to support a family on his own.
Ray