This is a very local question. Each city, county, state, whatever has a different process for appealing an evaluation. Contact your local office to find out what the procedure is.

I'd think that only the homeowner can appeal the assessment. He'd call and ask the assessor to re-measure the house.

Be aware that square footage is measured from the exterior of a house and includes all HVAC-ed spaces. If you're just adding up the dimensions of major rooms, you will come up with a lower figure.

He might have not subtracted out the garage space or the two story foyer.

Many tax appraisers include two story foyers twice.

They do not have access to the interior, so length times width time floors.

It may actually make no difference depending on the local tax appraisal rules.

It is not exact.

Would you sell for the tax appraisal?