“men of Nineveh.” The Greek word translated “men” is anēr (#435 ἀνήρ; pronounced an-air), meaning men; males. This verse is a window into the biblical culture. It would never have been considered in the biblical world that women would sit as judges, hence the men will rise up in judgment.
“behold” The Greek word is idou (#2400 ἰδού), and it is used to get our attention. See commentary on Matthew 1:20 (“Look!).
“someone greater.” The adjective “greater,” pleiōn (#4119 πλειῶν) is being used as a substantive (BDAG). As such, it implies a greater something, which is in this case a person, so “someone” is an appropriate noun to use to complete the sense. This is another time when Jesus revealed that he was the Messiah to those who had ears to hear. He was not simply saying he was a greater person or a greater prophet than Jonah.