Open the files 'Index.php' and 'Category_Dynamic.php' (located in the Store folder) and search for following line of code:

<?php echo tep_image(DIR_WS_IMAGES . $category['categories_image'], $category['categories_name'], 0,0); ?>

Replace 0,0 with the desired size in pixels.

Note that the code may appear in several different places in both files. The values must be updated in every location.