I'm using WP 3.9.1 and have added a gallery (Add Media ->Create Gallery -> pick images -> Gallery Settings: Link to Media File.) That works, almost.
The problem is that I need the gallery to use the custom size thumbnails I specified in my functions file. So I go into the text editor and change the gallery shortcode from
[gallery link="file" ids="672,540,537,556,557,541,558,535,555"]
[gallery size="thumbnail" link="file" ids="672,540,537,556,557,541,558,535,555"]
Adding the "size=thumbnail" works. However, it is not easy to do from the Media Manager / Gallery Editor. Am I missing something? I can't ask a client to remember to add this everytime. And what's worse is that any time you ADD photos (or otherwise edit the gallery), you have to switch to the Visual editor to make changes and when you click Update Gallery, it overwrites the "size=thumbnail" that was added. Is there a snippet of code I can add to my functions file to force the size?