Hi Alex,
The thumbnails are having a size of full width and it will adjust its width and height according to the number of columns you want to show on the shop page. For example, the width and height will increase if the columns you set are 2 and decrease if columns are set to 4.
You can set columns on Shop Setting’s option Number of items per row ie (Appearance -> Customize -> Shop Settings )
Let us know if you need further assistance.
Best Regards