@saunders you need to duplicate the file skins_fallback_v6/_item_content.inc.php (assuming you're using one of the b2evolution bootstrap-based skins) into your skin folder and modify the value of gallery_image_size to match the thumbnail size you want.

Regarding thumbnail sizes, you must use one of the already defined, or introduce a new one. More info here: http://b2evolution.net/man/advanced-php

@mgsolipa great, thanks!