Please read this article https://docs.woocommerce.com/document/fixing-blurry-product-images/ To change the size of the product images go to WooCommerce > Settings > Product > Display > Image settings and don’t forget to regenerate thumbnails after changes(use Regenerate thumbnails plugin).
Also, for your Shop page you can enable masonry (Theme Options > E-Commerce > Product Page Layout) then product will appear one below another without gap.
