Here’s what you can do:

Delete the Old Thumbnails

Download and install the Thumbnail Cleaner plugin. This plugin will delete all of your current thumbnails. You can do this manually by writing a PHP snippet that detects thumbnails and deletes them, but it’s not really that necessary.

Resize the Original Images

Once you have deleted the thumbnails, copy what remains of your uploads to your computer. Then, use PhotoShop’s automate feature to resize them all. Upload the folder back to your host, and overwrite everything. Ofcource, make a back-up first.

Regenerate New Thumbnails

After all the above is done, download and install the Regenerate Thumbnails by Viper00Bond, and create fresh thumbnails of your images.

Note:

Neither WordPress nor WooCommerce use the original images as product images or thumbnails. Unless you have no thumbnail at all ( which results in the original image being loaded ), there is no need to resize the original images.