I'd love to be able to do this for each exhibit template. Set an individual thumbnail size for each image so they can be irregular sizes, some quite large etc. Being able to set custom final image size for each image individually would also be great. Especially useful for different shaped images.
I guess it would it need a lot of code to customize. :)
I think it'd be possible to create a workaround by replacing each thumbnail on the server with different sized versions but not ideal when you want to change images a lot...