Have a look at the documentation here: http://learn.elgg.org/en/stable/guides/database.html?highlight=sizes#entity-icons

Be aware that this information is for Elgg 3. The icon sizes hook also exists in Elgg 2.x, but the thumbnails are only generated when uploading a new icon. In Elgg 3 they are generated on demand.

Thank you Jeroen

This mischievous hook on Elgg 2.3 works well --> elgg_register_plugin_hook_handler('entity:icon:sizes', 'object', 'set_custom_icon_sizes')  :)