If I use the Insert Media button to add an image to a post, I have four options for a link:
a. custom URL
a. attachment page
b. media file (this is always the full size file, which might be 3000 pixels)
I'd like it to link to a smaller media file, perhaps Large, or a custom image size. That would work well with my Lightbox/Colorbox plugin.
(I know I could find the URL of the custom size and paste it into the custom URL box, but I don't want to tell a client to do that. )