To change the resolution and compression:
1. Press the Func button.
2. Scroll down, if necessary, to the icon in the lower left corner of the LCD.
3. Use the right or left arrows to select L (large), M1, M2 (medium), or S (small). Press the Set button.
4. Use the right or left arrows to select superfine (quadrant with S), fine (quadrant), or normal (stairsteps). Press the Set button.
5. Press the Func button to exit.
Large/superfine images will take up the most space on the memory card. Small/normal images will take the least, but the picture quality will be less if you're printing the pictures.
There is a table at this link
(scroll down) to show you how many images will fit on a memory card at different settings.