This is really easy to do. The trick to achieving something unique here is to experiment. Try some really big sizes and consider decreasing the number of products per page at the same time, especially if you have a small niche range or sell products with interesting or detailed images. Here is how:
Hover over the Preferences tab and then click on the Images subtab. Notice that you can change the size of just about any image. This is really flexible and useful. Try experimenting with different image types.
Click on the Edit button next to whichever image you are going to mess with and enter your new size. Be daring, you can always change it back. Click on Save when you are done.
Now scroll to the bottom of the page and click on Regenerate thumbnails. Go and have a look at your products. Some of the images might need to be edited in your CSS file, and for some you will see the change right away.
Now, we can also reduce the number of products per...