you have to edit products.tpl and product.tpl.
look for something like this
Code:
{include file="product_thumbnail.tpl" productid=$products[product].productid image_x=$config.Appearance.thumbnail_width product=$products[product].product tmbn_url=$products[product].tmbn_url}
{$lng.lbl_see_details}</a>
you can change the parts
Code:
[product].productid image_x=$config.Appearance.thumbnail_width
to maybe
Code:
[product].productid image_x=100