How big is a 10-inch four-wheel ATV tire?

Small Square ATV Universal Tire is your good choice.
The "inch" referred to here should be inches
1 inch = 2.54 centimeters
10 inches = 25.4 centimeters
The tire size generally refers to the diameter of the wheel hub, that is, the diameter of the inner ring of the tire.
Plus is the thickness of the tire, estimated at 30 centimeters. Small Square ATV Universal Tire is your good choice.