The power store files are set to resize the image by width.
for example in the web assist/plugins/catalog/featured_box.php the following code sets the resize dimensions:
<?php
$imgWidth = 207;
$imgSrc = (($row_FeaturedRS)?'../../../'.($WAGLOBAL_Images_Folder).$row_FeaturedRS['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);
?>
maybe try changing that to use image height instead:
$imgHeight = 207;