There isn’t a “best size” as it depends on many factors such as;
- the size of your original images
- how many products per row
- the page width – the default container width is 1400px so if you change it to wider, then each column (products per row) will be wider than normal on larger screens
- the WooCommerce Product Image size that you have selected in the customizer
Have you looked at the WooCommerce Product Image thumbnail width in the customizer? The standard width is 300px unless you change it. You can try making this wider e.g. 350px or 400px and see if that works better.