What was the largest extinct venomous snake ever?

king kobra.

Well the largest extinct snake would be the Titanaboa which was as long as a bus however it was not venomous. The most venomous land snake according to the LD50 would be the Inland Taipan. While the most venomous of all snakes is the Hydrophis Belcheri or Belcher's Sea Snake. The last two are not extinct but the Taipan is very rare. The King Cobra is not extinct.