go to catalog/product_info.php
Look for
<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>
change SMALL_IMAGE_WIDTH and SMALL_IMAGE_HEIGHT according to your choice