With the WooGallery plugin you can easily set the gallery images thumbnails size as you want. Follow the instructions below: 

There are multiple pre-defined thumbnail sizes available in the dropdown. You can select any of them for your gallery thumbnails.

You can also set custom thumbnail sizes if you want. Select the option Set custom size from the bottom of the dropdown. Another option will appear after selecting it to set the height and width of the thumbnails. Insert the values as you prefer and go to the front end to see the changes.