Magic Fields wp_size parameter

I’m trying to use the wp_size parameter for the get_image function but I cannot figure out what is the value that is supposed to be passed here. I’ve tried all sorts of combos and cannot get anything to work.

I need to have the image height/width inserted into the ‘img’ tag so I need to pull those values from the media library. I could use the php function getimagesize() but if there is a way built in to this plugin to retrieve it from the library I’d obviously like to use it. Can anyone give me any suggestions on how to pass this parameter to the get_image function?