Look for this line of code:
Code:
{include file="product_thumbnail.tpl" productid=$recommends[num].productid image_x=$recommends[num].image_x image_y=$recommends[num].image_y product=$recommends[num].product tmbn_url=$recommends[num].tmbn_url id="product_thumbnail" tmbn_path_p=$recommends[num].p_image_path tmbn_link="false" tmbn_width="100"}
which is probably in the product.tpl file and the last part - tmbn_width="100" just change the 100 to whatever you want the width to be.