This is what DxO state for the Canon 1Ds Mark II
Sensor Size: 36 x 24 mm
Resolution: 5108 x 3348
Pixel Pitch: 7.05 µm
Firstly, the ratio of this sensor is specified as 3:2, but the resolution specified does not fit this 3:2 aspect ratio. Secondly, if pixel pitch is the distance measured from the centre of a pixel to the centre of the adjacent pixel (i.e. assume there are no gaps) then it can be assumed that to find out the size of the sensor you must run the following calculation:
Resolution x Pixel Pitch
This gives me a sensor size of 36.01 x 23.60 mm for this camera, which of course is different than what is specified by DxO.
So it seems there isn't an easy way to know how accurate this pixel pitch actually is? If I can assume it's accurate then I can easily just calculate the "effective" sensor size, for the 1Ds Mark II this would be 4992 x 3328 which would give me an "effective" sensor size of 35.19 x 23.46 mm. Or if running raw conversions through DCRAW which gives you more edge pixels so I would end up with an effective resolution of 5010 x 3336 which would mean a "effective" sensor size of 35.32 x 23.51 mm.
Interestingly digicamdb suggest a different pixel pitch. 7.21 µm:http://www.digicamdb.com/specs/canon_eos-1ds-mark-ii/