MacBook pro size

Actual size is 13.3 inches. See specs:

MacBook Pro (13-inch, M1, 2020) - Technical Specifications

Ofter, products whose size denotes a model have the value rounded for convenience. Note the title of the spec sheet.

Here are Apple's specs for both M1 and Intel machines:

https://www.apple.com/macbook-pro-13/specs/

Out of curiosity: why would it matter if it is 13 or 13.3"?