There's no single national minimum square footage for a commercial kitchen. In practice, size is set by three things: the equipment your menu requires, the clearances code demands around it, and the workflow your staff need to move safely.
Fit the required equipment (cooking line, refrigeration, the sink stack, prep and storage), then add code-required clearances — aisle widths, working space at panels, hood overhang and make-up air, and separation between raw and ready-to-eat prep. Ventilation and exit requirements can force more room than the equipment alone. The result, not a rulebook number, is your minimum.
Local zoning and building codes (occupancy, restrooms, accessibility) can add requirements on top. Confirm with your building department.