Directories Pro uses the following code to register the image dimension for cropped thumbnails:
add_image_size('drts_thumbnail', 240, 180, true);
and the following for scaled version of thumbnails:
add_image_size('drts_thumbnail_scaled', 240, 180, false);
and for icons:
add_image_size('drts_icon', 32, 32, true);
add_image_size('drts_icon_lg', 48, 48, true);
add_image_size('drts_icon_xl', 80, 80, true);
You can simply override them by adding the following code to functions.php of your theme (child theme) for example (320×240 px):
add_action('init', function() {
add_image_size('drts_thumbnail', 320, 240, true);
add_image_size('drts_thumbnail_scaled', 320, 240);
});
For medium/large sized images, you can configure them from Settings -> Media.
Make sure to regenerate thumbnails/icons after making the changes, using a 3rd plugin such as the Regenerate Thumbnails plugin.