Setting Image sizes
Since WooCommerce 3.3 the settings related to image sizes live in the Customizer.
To set the image sizes go to 'Appearance > Customize' to launch the Customizer and then navigate to 'WooCommerce > Product Images'.
Here you can set the 'Main image width' used for the main image on single product pages, as well as the width and the crop aspect ratio for the Catalog pages ( shop page & product categories ).
Thumbnails regeneration is handled by WooCommerce in the background but you can also regenerate them yourself using the Regenerate thumbnails plugin, if something in the automatic process doesn't work.
You can find the image settings under 'WooCommerce > Settings > Products > Display'. For any change to take effect you need to regenerate the thumbnails manually using the Regenerate thumbnails plugin.