Measuring in at just under 250 megapixels, the 19,580 x 12,600 APS-H sensor has the highest density of pixels for a sensor smaller than full-frame 35mm. When they put it in a camera, it was capable of resolving the lettering on an airplane that was 18km from where they were shooting from (which is insane). This isn't the first time Canon has announced a super-high megapixel sensor. They also introduced a 120 megapixel APS-H sensor back in 2010 — so this new one five years later essentially doubles the number of pixels in a sensor that's bigger than APS-C but smaller than full-frame.
Here's more from Canon on the sensor, which was just announced:
With CMOS sensors, increases in pixel counts result in increased signal volume, which can cause such problems as signal delays and slight discrepancies in timing. The new Canon-developed CMOS sensor, however, despite its exceptionally high pixel count, achieves an ultra-high signal readout speed of 1.25 billion pixels per second, made possible through such advancements as circuit miniaturization and enhanced signal-processing technology. Accordingly, the sensor enables the capture of ultra-high-pixel-count video at a speed of five frames per second. Additionally, despite the exceptionally high pixel count, Canon applied its sensor technologies cultivated over many years to realize an architecture adapted for miniaturized pixels that delivers high-sensitivity, low-noise imaging performance.
Video footage captured by the camera outfitted with the approximately 250-megapixel CMOS sensor achieved a level of resolution that was approximately 125 times that of Full HD (1,920 x 1,080 pixels) video and approximately 30 times that of 4K (3,840 x 2,160 pixels) video. The exceptionally high definition made possible by the sensor lets users crop and magnify video images without sacrificing image resolution and clarity.
Canon is considering the application of this technology in specialized surveillance and crime prevention tools, ultra-high-resolution measuring instruments and other industrial equipment, and the field of visual expression.
Five frames per second isn't good enough for more than the applications listed above, but I'm sure there are other binning modes that would still give very high megapixel videos at full video frame rates. Either way, it looks like in its current form we may only see it in a standalone camera designed for stills or for industrial video cameras that don't need 24 or 30fps.
If you're thinking that these things never make it into actual products, Canon just introduced a super low-light camera that started as a prototype sensor announcement in 2013, and the 50 Megapixel sensor that's in their 5DS and 5DS R likely started the same way. The biggest thing about these sensors is that even if we don't see these exact models in a product, the technology that they've developed does filter down into lower-end consumer applications. I would expect that based on how far this technology is moving, an 8K video sensor from the company isn't that far off — something we may even see in a high-end product in the next few years.