f you require more accuracy, you can do a "roll-out" test. Since the effective tire size is affected by tread thickness, tire pressure, and rider weight, the rolling circumference should be measured by rolling the bike with the rider aboard for one wheel revolution.
You may use the valve stem as a reference, starting the roll with the valve right over a perpendicular line on the floor, and ending when the valve is back at its low point one revolution later.
Another approach is to put a small dot of paint on the tire and measure the distance between the marks that the paint prints on the road. With either approach, the rider must hold the handlebars absolutely straight while an assistant balances and pushes the bike. Otherwise, the wheel may not follow a straight path.