get custom image size from option page is something that (should) work 😉
why? => because i had a site where i use that 🙂 (crated a few weeks ago)
but as john say try to recreate the custom size. (with help of plugin, or replace image with a new uploaded and look if that works)
if that not works, you may add the add_image_size at a place where option page is not able to access it, or something else is strange
