Open up the PowerGallery/galleries/grid_fullsize/index.php file and look for the line that looks like this:
echo '<img alt="" title="' . $i . '##' . str_replace('"', '\"', $gallery['images'][$i]['title']) . '##' . str_replace('"', '\"', $gallery['images'][$i]['description']) . '" src="' . $gallery['images'][$i]['thumb']['url'] . '" width="62" height="62" />';
If you haven't made any changes to this page, it should be on line 75. The width and height at the end of that line are what you would change to change the size fo the thumbnails.