Open product_info.php in whatever editor you have.... and look at about line 108, where it says:
CODE
<script language="javascript"><!--
document.write('<?php echo '<a href="java script:popupWindow(\\\'' . tep_href_link(FILENAME_POPUP_IMAGE, 'pID=' . $product_info['products_id']) . '\\\')">' . tep_image(DIR_WS_IMAGES . $product_info['products_image'], addslashes($product_info['products_name']), SMALL_IMAGE_WIDTH, SMALL_IMAGE_HEIGHT, 'hspace="5" vspace="5"') . '<br>' . TEXT_CLICK_TO_ENLARGE . '</a>'; ?>');
//--></script>
<noscript>
<?php echo '<a href="' . tep_href_link(DIR_WS_IMAGES . $product_info['products_image']) . '" target="_blank">' . tep_image(DIR_WS_IMAGES . $product_info['products_image'], $product_info['products_name'], SMALL_IMAGE_WIDTH, SMALL_IMAGE_HEIGHT, 'hspace="5" vspace="5"') . '<br>' . TEXT_CLICK_TO_ENLARGE . '</a>'; ?>
</noscript>
You can see where it sais the SMALL_IMAGE_WIDTH and SMALL_IMAGE_HEIGHT, and you should just be able to put fixed sizes in there instead.