Choose of other image size for the "enclosure" tag

[Former user]

Hello

I would like to know if there is a way to set up another image size for the "enclosure" tag? I use your plugin to set up a RSS feed used in Mailchimp for a newsletter and I need to different sizes of images: one is used from the "media:content" tag (which I can choose the size that I need), the other is used from the "enclosure" tag (where I don't see a way to modify the image size). Did I miss something? Why is it not possible to choose the size? Thanks in advance