Fly’s private registry is free for the time being: Registry @ registry.fly.io, is there cost to this?
I’ve seen Fly engs say even multi-gigabyte sized images are fine, ex: 2.2 GB docker image is not uploading. - #12 by michael
That said, for Machine apps, Fly intends to (it doesn’t currently?) charge in proportion to the size of deployed images: We're launching Fly machines today - #8 by starptech