This was my question to Tony Rocco, Building Code Specialist, Oregon Building Codes Division today. He gave me the following link:
Refer to Chapter 3, Section R304, “Minimum Room Areas.”
R304.1 Minimum area. Every dwelling unit shall have at least one habitable room that shall have not less than 120 square feet of gross floor area.
Habitable means sleeping, eating or cooking.
Update: The 120 square feet has been scraped. The new minimum is 70 square feet.