There may be formal limits but as long as you can carry it yourself, nobody is going to object. There is no checked luggage.

There seems to be stuff in the news back in March of this year saying only 2 suitcases per person, and if you take more you would be charged on the high speed trains. They did say a size but then they changed to unknown. I will attach the article.
Has anyone in the community traveled on a high speed Trenitalia recently with large luggage (30in suitcases)?

https://www.wantedinrome.com/news/italy-trenitalia-new-luggage-rules-high-speed-frecce-trains.html

I was announced by Trenitalia but after some protest they changed mind.

Trenitalia Website with the rules in italian:

https://www.trenitalia.com/it/informazioni/la_guida_del_viaggiatore/la_sicurezza_deltuoviaggio.html