On 4 November 2003, the largest solar flare ever recorded saturated the GOES X-ray detectors; from these a magnitude of X28 (2.8 mW/m2) has been extrapolated (http://sec.noaa.gov/weekly/pdf/prf1471.pdf). However, using the Earth's ionosphere as a giant X-ray detector, we show the magnitude of this flare is consistent with X45 ¿ 5 (4.5 ¿ 0.5 mW/m2), or more than twice that of the two previously recorded largest flares, both about X20. This flare magnitude is determined by using the large observed phase changes recorded at Dunedin, New Zealand, on long VLF radio paths across the Pacific from transmitters in the continental USA and Hawaii. The enhanced X-ray flux caused a dramatic lowering of the height of the D-region of the ionosphere, allowing the flare to be measured relative to the GOES observations.