"You are not big."
Duo is just reminding there is another word order. Very often we have to accept marginal wors orders and it is useful to see those more natural or common ones.
A native speaker should know that in Czech theree are deep diffeeces between different word orders given by https://cs.m.wikipedia.org/wiki/Aktu%C3%A1ln%C3%AD_v%C4%9Btn%C3%A9_%C4%8Dlen%C4%9Bn%C3%AD (https://en.m.wikipedia.org/wiki/Topic_and_comment). This is also true hre. Nejsi velký and Velký nejsi ar both perfectly valid but are not the same.
Both are grammatically correct, I think it is a matter of emphasis; the meaning and tone of a sentence often revolves around the last word. You are not big vs. big you are not, in this case?
Short answer: In this particular sentence, the gender and number are not specified, so any of the three forms you listed (plus velcí) could be used.
Longer answer, for those who may want more help with adjective/noun pairing: The adjective needs to match the noun in gender, number, and case. So the place to start is with the noun: Masculine animate, masculine inanimate, feminine, or neuter? Singular or plural? What is its role in the sentence (for case)? You can see the full declension table for velký at this handy site: https://en.wiktionary.org/wiki/velk%C3%BD.
(Edited for easier access by users at different levels)