Generally speaking, in order to make a product image look clear, you have to upload images no smaller than 300*300 pixels.
However, sometimes, the
optimum size for product images depends on the theme you are using. Once you have activated a theme, you can go to the "Design"->"Theme"-"Theme Settings"->Image Size to check the optimum size for the theme you are using. 

The size of the image you upload should be no smaller than that size (if it's bigger, remember to keep the same length to width ratio). Also, you can choose not to use the default image size of the theme and change it by inputting the image size into the "px" box.