Does anyone remember the specifications of how long a witcher's sword should be?
I think I remember someone mentioning it at one point in the game, early-ish?

I think
this should be enough

Hattori mentions something about Geralt's being about 40-inches long, if my recollection is trustworthy...

I don't think you need any specific information from an NPC in-game to answer your question. Geralt can wield his swords both, with one hand, and with two hands. That indicates that all his swords are so called 'Bastard Swords'. Ask Google and you will get your answers. Most probably there were small modifications. The blade might be double edged and suitable for stabbing. That would make it quite heavy. Not an expert here :P

I guess Yennefer would be perfectly able to provide the answer here.
Or, just ask Keira - she knows for certain :).

The witcher sword Geralt has made for Ciri is 38 1/2 inches.