How to post custom image size?

Hi,
I’ve been using this plugin for a few days, and i have to say it’s awesome. i will probably buy the pro version soon. However, i have a problem (more of a situation) that i need to solve.

Featured images of my posts are mostly huge (8000×5000) and aren’t watermarked (i add the watermark to thumbnails). This plugin is using the original featured image link to post it into the social media networks.

I would like to use the ‘medium’ size image as the posted image on the social network. is it possible to alter the arguments of nxs_getPostImage() to get ‘medium’ sized image instead of ‘full’ image?

Any help is appreciated.