I don't think it would be productive to try to come up with a precise measurement for this, since it's, at base, a matter of what's reasonable to expect the community to create and maintain using the SE toolset.
There's guidance in the official description of the "Too Broad" close reason that provides what is probably a sufficiently precise limit for the size of answers that a question is allowed to require: "can be answered in a few paragraphs."
The biggest issue, as I see it, with questions that require answers larger than a few paragraphs is that we depend, for essential quality control, on peer review by a community of volunteers who choose on the fly what to review. Our most basic review mechanism is the vote, which is, by design, lightweight, anonymous, unaccountable, and available to nearly everyone in the user community. Our expectation is that for any given question in our scope, a significant number of community members can and will review answers to that question with at least sufficient rigor to vote fairly on it. The larger answers get, the lower the likelihood that users will choose to devote the necessary time and attention to give them fair reviews.
Note that I am only addressing here the issue of size of required answers, rather than the issue of number of possible answers, although the issues are related. The former is what this question post asked about, and it is the issue with the Mi Yodeya question that prompted it. There was a proposal, in the chat about that question, to convert it into asking for one list item per answer rather than one answer containing all list items. That proposal would put the question into "too many answers" territory without really rescuing it from "too large required answer" territory, since conceptually, what the question would still actually be seeking is an exhaustive list that is necessarily much larger than a few paragraphs.