I see, I would include sponsored persons and include a letter explaining that sponsored person added every year for income calculation purpose.I am referring to Financial Evaluation form IMM5768e. Different information sources of IRCC state different things about family size being the same as total number of dependents for income purpose. TO be safe, I suggest having immediate family in the table that requires three-year income and making a note that the family size does not include the person been sponsored. If they want, they can add this person themselves.
I think that is the main purpose of family size.