Asked the Same question on another page and was surprised at how close it was. They both have similar number of trophies.
They both have 19 League titles and Milan has 1 more Champions League.
You could argue Milan has more iconic legends and a more famous stadium and that in their peak were more desirable for players. Liverpool on the other hand probably have more fans (social media) and are worth a few billion more in value.
Few criteria when arguing about who’s the biggest club, but for me, fanbase and global fame are the most important.
Who is more well known in India, America, or Africa?
Liverpool is more globally famous than Milan, but Real Madrid, Barcelona, and Manchester United are the 3 biggest.
This answer changes probably every decade or so. I’d say we’re both on a tier below Barca and RM. both have huge illustrious histories. Both have had supremely dominant periods domestically and in Europe.