HEAD requests seem like the only option aside from inferring size from suggested resolution and format. Imagine trusting the author to include the resource size.

If I could add to the spec, I would put something for lofi/hifi. Then of course that is all subjective and developers will eventually use it in problematic ways.

There's already enough gaming around pagespeed scores. If such an incentive is added, the spec needs to be formulated in an ergonomic way to avoid mal-optimizations.