I got the same issue.
I edit the file of JA Slideshow , and finally got the larger size of image I want.
at the line 406 of /helpers/helper.php
//return $arrImages['imageGeneric'];
$row->mainImage = str_replace(JURI::base(), '', $arrImages['imageGeneric']);
$row->thumbnail = str_replace(JURI::base(), '', $arrImages['imageGeneric']);
to
//return $arrImages['imageGeneric'];
$row->mainImage = str_replace(JURI::base(), '', $arrImages['imageXLarge']);
$row->thumbnail = str_replace(JURI::base(), '', $arrImages['imageGeneric']);