When Quenya adjectives qualify plural nouns, they are pluralized by the well-known pattern (a > e, e OR C > i); but when an adjective is the predicate of the clause (with the copula either present or not), the attested examples may suggest their plural forms follow those of nouns.
I have found three examples of adjectives ending in a:
Eldar alphírimar “Elves are Immortals”
ilya sí maller raikar “now all roads (are) bent”
Sindar i Eldar Malariando “Grey are the Elves of Beleriand”
There is one example of an adjective ending in e which is of course not conclusive:
i roccor rindi “the horses are swift”
I have also found two examples that seem to refute this theory however, it may be relevant that both examples are with mára, so it's possible that the usual plural form is used to avoid two Rs close to each other1:
qentien no máre nar i hondor “*than good, I said, are the hearts”
roccor i Erulingaron márë nár (ma naitë)? “the horses of the Rohirrim are good (are they not, lit. is it true)”
There is also this attested question:
malkar i·aldar i·ólar nóressella “how great are the trees that grow in your land?”
Arguably, in this sentence, the interrogative malkar stands in place of the predicate ("how great are..."), hence its form malkar; it's possible, that when the same interrogative is in an adjectival position, it would take a plural form usual for adjectives:
*Malke aldar tarir ara i koa? "How big trees stand beside the house?"
- 1. This argument may of course be spurious as in hárar the two Rs are apparently not a problem