How to make WP Gallery use media library image sizes

Hi there,

Is there something we can do to make the WP gallery use the media library's large or medium image size instead of the full size image?

I'm building a site and I have some nice high res images to use. But I'd rather not have to process them all before uploading them, especially as WP already produces images at the right size - the large image size would be perfect for me, but it could just as well be other sizes in other situations.

I just don't need the galleries to use the full size image.

In this case I'm using the Gallery Slideshow plugin, but I don't think that is a particular hindrance in this situation.

Is there something we could add to functions.php or something to make the WP gallery use our selected image size?

Note: I am not talking about the size of the gallery thumbnails that you click to see the bigger image; I am talking about the size of the bigger image that is displayed once you've clicked the smaller thumbnail.