How can I change the product image size?

The product image size and other related settings are configurable from the app preferences page in the Visual Preferences section:

In the following image you can see where those preferences are applied:

The app will look better if the configured width / height ratio matches your images width / height ratio. For example, if you images are square, set the same value for width and height. Or if your images are 4:3, use proportional values such as 160 and 120.