How big is the fuel tank on a 1998 Chevy Suburban you were told it was 40 gal but put in 42 last fill up?

Putting in 42 gallons when the gauge indicated empty suggests there might be some inaccuracy with the fuel gauge, or perhaps additional capacity beyond what the manufacturer's stated tank size is. There could also be a small reserve not reflected in the gauge. It's unlikely the tank itself is actually larger than 33 gallons.