Ensuring accurate metering in the context of commerce is within the scope of "To … fix the Standard of Weights and Measures". The specific form of the nozzle clearly is not, but they might justify it on the basis of some other enumerated power—the interstate commerce clause is frequently (ab)used for this sort of thing. Nothing technically requires every regulation produced by the NIST Weights and Measures division to be grounded exclusively in the Weights and Measures clause, though one could be forgiven for making that assumption.
As dpratt remarked earlier[0], any interpretation which would deem nozzle size—or the specific form of an electrical connector—to be covered by the Weights and Measures clause of the Constitution would effectively cede unlimited power to the federal government. What couldn't they regulate under such broad rules?