According to U.S. Code, the laws of the United States, Title 4, Chapter 1, Section 1: § 1. Flag; stripes and stars on Release date: 2005-05-17: The flag of the United States shall be thirteen horizontal stripes, alternate red and white; and the union of the flag shall be forty-eight stars, white in a blue field.
I found no size requirements for the flag, though there are several laws governing its display. The requirement about the number of stars and stripes is the closest thing I found to the physical make-up of the flag.