"didn't specify how large the sensor actually is"
According to http://www.symbian.org/devices/nokia-n8 and Nokia Conversations website, it's 12-megapixels.
Nokia has introduced its first Symbian 3 smartphone, the N8. The handset's headline feature: a 12Mp camera that the Finnish phone giant claimed can "rival dedicated point and shoot cameras". Nokia N8 Nokia's N8: compact camera-beating snapper? Key to that claim are Carl Zeiss optics, a xenon flash and a "substantially" …
I believe the question refers to the physical size of the sensor, not the resolution. The importance being that a larger physical size is better, for a fixed resolution.
For example, see http://www.dpreview.com/news/0210/02100402sensorsizes.asp
The answer to this question seems to be 1/1.9" which, according to the following page, is quite impressive:
http://blog.gsmarena.com/how-large-is-the-nokia-n8-12-megapixel-large-image-sensor/