Should developers provide a size for their registered content?

[rayankans]

It would give users a better idea of how much space they would get back upon deleting an entry, and makes for better UI.

[quasicomputational]

Wouldn't any entry-deletion UI be provided by the registering website, rather than the browser? I can't see how the browser would know the set of resources that could be freed. And, since the website can presumably keep track of this data along with the resource dependency graph, I don't think this needs to go in the registration API.

[rayankans]

Wouldn't any entry-deletion UI be provided by the registering website, rather than the browser

Ideally the website should have an entry-deletion option, but it must be provided by the browser.

Browsers are encouraged do display the offline-enabled content to users, and there will likely have to be an option to delete the listed contents, since there's no guarantee that a website will provide that option. At that point users will expect to not see that article (or whatever type of content it was) surface again.
The size of the underlying content is usually a factor in these decisions. The browser can also at that point check if the amount of storage freed is roughly equal to the value provided in the registration, to make sure websites clean-up the underlying content when a registration is deleted.