Click on this link to see an example:

http://www.dpreview.com/reviews/compare_post.asp?method=sidebyside&cameras=canon_g7%2Ccanon_g9%2Ccanon_g10%2Ccanon_g11&show=all

It is the specifications for one camera over four years.

Look down the left column until you see "Effective Pixels"
That is how many pixels on the CCD sensor assembly.
Click on the yellow ? just to the right of "Effective Pixels",
and you will see an explanation.

Next look two lines down until you find "Sensor Size".
That gives you the size of the CCD sensor assembly.
Click on the yellow ? for an explanation.
At the bottom of that page click the yellow "Sensor Size" to
see the differences between sizes.

Next line down is "Sensor Density".
That is how crowded it is with that amount of pixels.
Click on the yellow ? for an explanation.

Notice that the G9, G10, and G11 use the same size CCD sensor assembly.
But each one uses a different amount of pixels.
Notice that on the G11 they lowered the amount of megapixels to 10.
That let them use larger pixels.
A larger pixel can gather more light.
That lets the camera work better in low light.

...

To find the sensor size for any camera:
Go back to the link I provided and look in the far upper left corner.
Hold the mouse cursor over "Buyers Guide.
In the pop-up, click "Side-by-side"
From that window that opens you can select cameras and move them to the right side and then do a compare to get the specification charts similar to the ones I chose in the link I provided.