- You can actually have up to 4 or 5 sizes right now, depending on your UHD settings. It comes from the &dpr=M in the asset URL, where M is the device ratio multiplier. For these, we don't like to generate them unless someone actually needs to see it (they are only used in the JS lazy-loader until we add srcset support).
- There's no interface for it, but under the hood, the idea is to allow for arbitrary sizes that can be approved on a case-by-case basis. Generating these for every image would be excessive.
* A rebuild job from the admin panel has the drawback of only rebuilding one image at a time (each one is allowed to take 5 seconds so it could potentially take hours); meanwhile, "on the fly" we can do multiple images at once and focus on the images that people are actually requesting first.
We don't do a post save task in the same request due to resource limitations, but we could definitely add an immediate deferred task on post save that generates the base thumbnail (assuming someone hasn't already tried to view it in that second).