Mailman3 supports fulltext search, a long-requested for mailing lists. There are multiple backends which are supported. For now I'd like to avoid elasticsearch since it adds in an extra dependency on another service, so realistically we're looking at xapian or whoosh (pure Python).
xapian is currently enabled and rough estimates look like it'll be a 212 GB folder. (From T278609#6982087) but that estimate might be inflated because of attachments. IIRC whoosh is a smaller index but slower indexing.
Of course we should also look at quality of search results.