How To know if Image Size is: Thumbnail, Medium Large in Gallery Settings

Hi,

I’m using Masonry with my wordpress theme and I want to be able to know if what kind of size a user clicks on the radio button in the gallery settings so that I can input that in the class. What I’m currently doing is putting the ‘medium’ and ‘large’ in the title and I would get that using ‘post_title’ and that works just fine but it would be easier if I can just click the radio buttons.

For example, I tick the Medium radio button in the gallery settings for one image and the other image I would click the Large radio button. I would then input it on my gallery shortcode so that my masonry layout have two different image sizes, one is large and one is medium.

in my functions.php, I would echo it like so:

echo '<div class="item ';

           //Where [thumbnail_size] is the title of the thumbnail size.
           echo $attachment->[thumbnail_size];
       echo ' ">';

I know this:
image title = post_title
image description = post_excerpt

but what about the thumbnail size.