In 1492 (and really up until very much recent times), there was just so little standardization of languages for this to be at all a simple question. I personally am opposed to "Italian" being included as a language in this hypothetical list of the largest languages at the time mainly due to the lack of a common, standardized, prestige form of the language, and the extreme gulfs of difference between the different languages of Italy (Sardinian as opposed to Tuscan as opposed to Venetian as opposed to Neapolitan). Arabic definitely has similar linguistic diversity, heck some people today in Morocco consider their dialect of Arabic as a completely distinct Berber-influenced language, but the big difference is the commonality shared by the relatively standardized Classical Arabic of the Qur'an which was used as the standard throughout the Islamic world and the Arabic speaking world. Arabic is definitely in the list of the largest languages.
I also agree with including Nahuatl- mesoamerica was one of the most populous regions of the Americas (another very populous region, the Mississippi, didn't have a single language as dominate as Nahuatl was in Mexico). Nahuatl sort of has a similar issue as Italian in that there's lots of smaller, regional dialects (should Pipil be included as Nahuatl?), but the upper-class Classical Nahuatl that was used by the nobility in Mesoamerica functions as a sort of unifying force that I could see used to include it like Arabic.
I don't have any specific numbers to actually base this on but in no particular order I think it is safe to assume the following languages would be included:
Han (Mandarin) Chinese
Wu Chinese
Arabic
Hindustani (sizable dialect continuum but the dialect of Delhi has already been pretty established in influence by 1492)
Bengali
French (Langue d'Oil)
German Dialects (though the rapid standardization of German falls out of this year, Germany is very populous and the beginning of the standardization happens soon after)
Nahuatl
Would any Turkic or African languages be included or are those regions just too sparsely inhabited? I'm not totally certain about Greek, Turkish, and Ruthenian/East Slavic but they could probably also be included. Persian probably would have been included if those dang Mongols hadn't happened just 200 years earlier, followed by Plague and then Timur right after.