At this physical size, Canon's 250-megapixel sensor has a pixel size of around 1.5 microns, which puts it in the realm of smartphone cameras. The light gathering capabilities of this sensor will be significantly lower than Canon's high-end DSLRs, such as the 5D Mark III and its 6.25 micron pixels, but that's not the point of a sensor with this many pixels.
Instead Canon is touting this sensor's capabilities in surveillance and crime prevention tools. Paired with an appropriate lens, Canon was able to distinguish the lettering on the side of an airplane flying 18 kilometers away, which is a pretty impressive feat.