Hello,
To adjust the size of the product thumbnails, open config-woocommerce > config.php then find this code
$avia_config['imgSize']['shop_thumbnail'] = array('width'=>80, 'height'=>80);
$avia_config['imgSize']['shop_catalog'] = array('width'=>450, 'height'=>355);
$avia_config['imgSize']['shop_single'] = array('width'=>450, 'height'=>355);
If you want your lightbox not to crop the images on load, open up js > prettyPhoto > js > jquery.prettyPhoto.js and find this code
allow_resize:true,default_width:500,default_height:344
Set allow_resize to false.
Regards,
Ismael