Default Thumbnail Size

Hello,
I have the plugin set up to override standard WP galleries and display the images in the Basic Masonry format. How can I control the default thumbnail size for the masonry grid? Right now it seems to be loading the medium size (300px). I’d like it to load a larger image size (ie medium_large). I can see the setting where I can control that for an individual gallery. I just can’t figure out how to update the defaults.

Thank you.