in the webassist/plugins/catalog/popularbox.php file, the following code generates the iamge:
<?php
$imgWidth = 207;
$imgSrc = (($row_MostPopularRS)?'../../../'.$WAGLOBAL_Images_Folder. $row_MostPopularRS['ProductThumb']:'images/default-product_thumb.png');
$imgSrc = abs2rel(rel2abs($imgSrc,dirname(__FILE__)),dirname($_SERVER['PHP_SELF']));
$WAResultsImage = new WA_Include('scripts/resize_image.php');
require($WAResultsImage->BaseName);
$WAResultsImage->Initialize(true);
?>
<a href="../../../products_detail.php?ProductID=<?php echo($row_MostPopularRS['ProductID']); ?>">
<img src="<?php echo(abs2rel(rel2abs($WAResultsImage->Body,dirname($_SERVER['PHP_SELF'])),dirname(__FILE__))); ?>" class="popularImage" />
change the $imgWidth = 207; setting to change the image dimensions.