It is hard to say when Seax started, but certainly the curved edge and spear point styles were around from the 7thC. and the classic broken back (near) straight edged form was around from the 10thC.
They came from the small at 3-4” to the huge at 26” or more, so as a basic pattern they were small personal knives through camp knives, daggers and up to swords.
Construction was basically the same in that the tang did not pass all the way through the handle and was bonded in and there was very rarely a guard or a pommel.
Blades were often pattern welded.