What is the largest Entry Foyer permitted in an Apartment?

Q: What types of spaces may be declared as a Foyer, with no Windows? What is the maximum size of an Entry Foyer?

A: Entrance Foyers to Dwelling Units do not require Legal Windows provided that they comply with MDL , Title 3, Light & Air, Section 31.5, excerpted below:

5. A portion of any apartment used as an entrance hall to such apartment may be designated as a foyer. Such a foyer shall not be considered a room if the department shall so permit and if either:

a. Its floor area does not exceed ten per centum of the total floor area of such apartment, or
b. Every room in such apartment exceeds in area the minimum required area of such room by more than twenty per centum and the floor area of such foyer does not exceed twenty per centum of the floor area of such apartment.

An entrance foyer is not a Code Habitable Room. Definition of Code Habitable Room is below from NYC BC Definitions:

HABITABLE ROOM:
A residential room or space, having the minimum dimensions required by §27-751 of article six of subchapter twelve of this chapter in which the ordinary functions of domestic life are carried on, and which includes bedrooms, living rooms, studies, recreation rooms, kitchens, dining rooms and other similar spaces, but does not include closets, halls, stairs, laundry rooms, or bathrooms.