Shopping for rotors, you will see measurements like "11.6 inch" or "296mm" - but rotor sizing involves several dimensions, not just one. Getting them all right is what makes a rotor actually fit and work. Here is what the numbers mean.
Diameter is the headline number
The most-quoted rotor dimension is the outer diameter of the friction ring, in inches or millimeters. It must match your vehicle's spec exactly - the caliper is positioned for one diameter. A larger-diameter rotor gives more leverage and cooling, which is why performance and towing setups sometimes upsize, but you cannot simply fit a bigger rotor without matching caliper brackets.
Thickness and minimum thickness
Just as important is rotor thickness, and its companion the minimum (discard) thickness stamped on the rotor. Rotors wear thinner over time; once they reach the minimum they must be replaced, not reused - see rotor minimum thickness and how to measure it. Thickness also affects heat capacity and resistance to warping.
Hat height, bolt pattern, and centering
A rotor also has to match the hat height (how far the friction ring sits from the mounting face), the center bore, and the lug bolt pattern. Get any of these wrong and the rotor will not sit flat or the wheel will not clear. This is why rotors are sold by exact vehicle fitment rather than diameter alone.
Bigger is not automatically better
For normal driving, the factory rotor size is engineered to match your car's weight, caliper, and wheels - oversizing adds unsprung weight and cost with no everyday benefit. Match the correct size and focus on quality. A properly-sized rotor with good pads and fresh hardware outperforms a mismatched big rotor every time.