so this is what I would do:

yes :-)

I cannot see why this would be a performance drag, because this only happens once and not with every query.
Also keeping the size up to date would be only necessary when the size changes.

use case:
When I'm reading a page in f.i. german with very few information it would be interesting to see if there is a linked page in another language that provides more information (maybe up to 10 times more).
If this was english I could read it right away, if in another language I could translate it with google-translate (which is better than having no additional information at all).

Yes, the cost would be a problem.
But would it be possible to add a new field "ll_size" to the table "langlinks"?
Then it would be very fast...

After adding an "s" to revision it works, but it only gives me 1 size, probably of the first language:

thank you for this, but when I enter your URL
https://de.wikipedia.org//w/api.php?action=query&format=json&prop=langlinks|revision&llprop=langname|url|autonym&rvprop=size