Indicate image size to use as gallery popup?

Is there any way to tell the standard WP gallery a particular image size to use for the image that pops up when you select a thumbnail image?

Background: Galleries have already been set up on a client’s site using VERY LARGE images (of all different sizes). I’ve gone through and created a new image size in functions.php (let’s call this new size gallery-full).

I also regenerated images (by using the Regenerate Thumbnails plugin), and if I go to add an image to a post I see my new gallery-full option as one of the image options.

Instead of having to go back and replace all the existing images with resized versions, I’d like to be able to tell the Gallery logic to use the gallery-full image size (for the particular image that’s currently listed).

Is there some way I could do this via functions.php, perhaps? Or some other way (that won’t get wiped out by core updates)?

Thanks.