It was 1Gb initially, then on their 1st birthday 2Gb and slowly growing daily, which seemed practically unlimited compared to other email that were offering 100Mb at the most? A quick look at an article[1] at the time (who claimed it was "outrageous") show Hotmail were offering 250Mb and Yahoo had just launched 1Gb to catch up.

[1] https://www.searchenginejournal.com/gmail-rewards-users-with...