I believe you are referring to the home office calculation. The maximum allowable is 300 square feet so if you have less than that, you use the square footage you actually use (e.g., your home office is 250 square feet, the maximum allowed is 300 square feet, thus you would use 250).
See https://www.irs.gov/businesses/small-businesses-self-employed/faqs-simplified-method-for-home-office...
