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 Styles section:

Once there, enter the  Product image preferences section by clicking on the menu or on the item you want to customize, in this case, the image of the products.

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

According to the example configuration, the images now have the height of 212px, and the width will be automatically adjusted to the exact width/height ratio of the original image if the scale option is enabled.

If the scale option is unchecked the width will not be automatically adjusted, but in that case, you can manually adjust it by manipulating the width and height preferences. Please take a look at the related articles for more details on this matter.