Hi Scratch,
They vary a lot, but most fall between 11-12 inches total length. There are many distinct styles and blade shapes. The wider blades like this example are very heavy for their size. I have a few that weigh over a pound and a half. I believe Artzi has one even heavier than that.
Hi Jim,
I like the comparison to the scramasax. I've often looked at the old scramasax blades and wondered what the entire piece looked like in its best day. You know, celtic/germanic/norse art used the kinds of intricate, organic patterns that were similar to sinhalese art. Imagine a scramasax decorated like a piha! Check this article for fun:
http://www.myarmoury.com/feature_seax.html) It sort of goes against your assumptions, but seems entirely possible. In fact, if you a piha degraded down to just an old oxidized blade and tang, you would be hard-pressed to imagine it in it's presentation condition.
-d