in cart.tpl find this line...
Code:
<a href="product.php?productid={$products[product].productid}">{if $products[product].is_pimage eq 'W' }{assign var="imageid" value=$products[product].variantid}{else}{assign var="imageid" value=$products[product].productid}{/if}{include file="product_thumbnail.tpl" productid=$imageid image_x=$config.Appearance.thumbnail_width product=$products[product].product tmbn_url=$products[product].pimage_url type=$products[product].is_pimage}</a>
change the part that says " image_x=$config.Appearance.thumbnail_width" to "image_x=100" (or whatever you want the width to be in pixels)