So large a head and so large a number, both singular, are possible, but is so large people also possible, or does this construction not work for plurals, (meaning only such large people is correct)?
In a comment, John Lawler wrote:
That construction doesn't work for plurals, because they don't use indefinite articles. Such is normally used for nouns: Such a large head, such large heads, such colorful flooring/azaleas, etc. So is for adjectives, normally only as predicates: So beautiful, so hot, so pregnant, but the special construction allows the so large part to appear before an indefinite noun phrase a head; it's an idiom. So large a head means the same thing as such a large head. And both of them can refer to a complement clause -- so large a head/such a large head that he can't find a hat to fit.