Occasionally when learning gemara one comes across an incredibly long and daunting Tosfot like s.v. Parah on Bava Kama 76a or an "iceberg" such as s.v. Mah on Yoma 82a-b. (These are just examples.)
So, what is the longest Tosfot in Shas?
 You can't see the majority of an iceberg, as it's underwater.
Edit: if people post serious suggestions in the comments here, I can analyze word counts for individual tosfots. But please make it your best guess after considering the suggestions already made. And please link to the daf on hebrewbooks.