The preposition da does not take an accusative. The accuasitive is used with preposition only to distinguish locatioon and direction with those preposition which can give both:
en la domo – in the house (location)
en la domon – into the house (direction)
but: al la domo – to the house (direction, but the preposition al can give directions only)
Because there was a big fight about 70 years and tiom-kiom-ismo pretty much lost following an argument to Zamenhof's usage. As a result:
Kiom should (properly) be reserved for actual expressions of measurement or number, including with a da phrase.
This is one of the very few cases of prescriptive grammar in the language.
I had not heard of tiom-kiom-ismo (and I would love it if you could cite a source or link for the "70 years" number.) I do know, however, that it's possible to use tiom and kiom without da -- and for grades, so I decided to look for a source for tiom-kiom-ismo.
I found this (see link below.) I think you're mistaken. "Tiel granda" and "tiom granda" mean different things, but both are valid expressions.
Edit - let's degree to disagree - har har har. :-)
"Du malsanoj en Esperanto." It's not authoritative, but the arguments are clear and convincing to me. (The other malsano is using "kia" like "ol" and "kiel".)
I hesitate to just call a grammatical structure "emphatic" -infrequent structures usually emphasize some specific aspect of meaning. Substituting "kiom" emphasizes quantifiability, like when Z uses it: "
La propagando de Esperanto, kiom ajn grandaj estas ĝiaj lastaj progresoj, estas apenaŭ komencata."
De Hoog's observation is that Zamenhof scarcely ever did this and his main argument against tiom-kiom is that its popularity creates a style where it becomes hard to understand the limits of tiel-kiel, confuses new Esperantists, and departs from the Fundamento. Although it's a prescriptivist position, it doesn't say that comparisons and analogies with kiom are always wrong, only that they stretch the meaning of kiom and confuse the usage of kiel.
I think our disagreement is mostly in degree. I wouldn't introduce kiom as a more emphatic option when comparing degree. I'd say that it makes a quantitative metaphor - it denotes that someone could count or find a ruler or a scale - and that this sense is sometimes (rarely by Z) stretched to qualitative statements for rhetorical effect.
Thus I understand that, "kiom longa" is perfectly natural for measuring timber, "kiom granda" may be natural or stretched depending on the sense of "granda", "kiom aminda" is certainly silly - but poetry may benefit from silliness.
As jxetkubo alludes to in his later replies, "kiom" in place of "kiel" here does not precisely mean the same thing. There's a sense of "emphasis." That is, you're kind of saying "yes, I know the universe is huge, but how huge is it?" With "kiel" it's more neutral - like "what is the size of the universe."
Neniu scias kiel granda la tuta universo estas, sed la observebla universo estas proksimume 13.8 miliardoj lumjaroj.