Your camera has three .jpg compression settings.
Superfine, Fine, Normal
If you need to get more photos on a card, you can set the .jpg quality setting to Fine.
In most photos, you will be unable to tell any difference between superfine and fine.
If you need to get even more on a card, then also set the size to 2592 x 1944 (5 megapixel).
I suggest you take three identical photos,
one set to superfine, one set to fine and one set to normal. See you if can tell any difference.
A photo that includes lots of surfaces, such as near-by grass and tree leaves would be best for the test.
I have a new canon s80. which i really love. I am going on a trip this summer and i would like to know more about image size vs. quality...so if i wanted the highest quality pictures i will select large and superfine...but say i need a little more efficiency out of my card, what are the advantages and disadvantages to 'superine and medium' vs. 'fine and large'....i probably wont be blowing any picture up bigger than an 8x10 ever....thanks for the advice!!