Board Thread:Warhammer 40k General Discussion/@comment-176.205.23.248-20140531233700/@comment-4851119-20160424112431

The Beast is canonically the largest in recorded history IIRC, Blackfang and the Mekboss of Gorro are close seconds. The biggest currently operating is of course Ghazzy.

The Krork warmasters of the War in Heaven would probably have been bigger than them but we know nothing about that era so we can't confirm.