That image-size is being controlled by your template's responsive.css, line 13:
Code:
img{max-width:100%;height:auto;border:0;}
I had success changing the 100% to 200%, but if you do that you'll want to create a separate, page-specific, CSS file named products_all.css in your template's /css directory. In that file, use the following (and edit for the required sizing):
Code:
#allProductsDefault a img { max-width: 200%!important; }
You need to !important that style, since the responsive.css loads at the very end and will cause any change you make in that page-specific styling to revert to its styles.
P.S. You should also address the HTML validation issues on that page.
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