All my luthier tools are kept in a 19"/48cm Stanley plastic tool case, for now at least. I'm going to need another as I've just got some more radius blocks which make it all of a squeeze. And finding stuff when it's full isn't that easy. So don't overfill it. I have a few of those Stanley boxes, but I won't get any more as the plastic hinges snap too easily, or at least parts of the hinge break off. Still allows you to fit and shut the lid, but it keeps falling off so you have to refit it every time.

I probably wouldn't recommend going smaller than 19"/48cm, as a few of my tools wouldn't fit in anything shorter. But obviously you need to be able to store it conveniently, so don't go too big if you are limited for space.