under the galleries folder is a subfolder for each different gallery.
in each of those folders is an index.php page.
in the index.php file, there will be a line similar to:
php:
$gallery = get_gallery(get_mediarss_url('gallery=' . $pg_id . '&thumb=132c&full=782p'));
that initializes the gallery and tells it what size the images should be.
this portion:
thumb=132c&full=782p
tells it to use the 132 px wide cropped image for the thumbnail and the 782 px wide proportional image for the large image.
the available choices are:
proportional:
768
481
92
cropped
62
140
102
42
481
260
