"Horses are big."
Translation:Konie są duże.
In that case it would be "duzi", because "mężczyźni" is masculine personal plural.
The ending of adjectives which are masculine personal plural (ie. a group containing at least one male person) is -i with the preceding consonant being "softened" (in that case "ż" becomes "ź", which is written just "z" before the letter "i"), and the ending of other plural adjectives is -e.
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Even if that was correct, the right form would be "dużymi".
What you describe happens for noun phrases ("Horses are big animals" is indeed "Konie są dużymi zwierzętami") but if you have just an adjective on its own, it stays in the case of the rest of the sentence, so usually Nominative ("Konie są duże").
You can read more about it here: https://www.duolingo.com/comment/16373167