With the volume in mind, remember that the 180L modern playboats are
very different boats from what you are looking for. They have a flat
bottom, they are very hard to paddle straight (without experience), and
the ends can dive underwater.

That being said, 360L would be fine (I think). An older whitewater
kayak certainly had less volume and never had any buoyancy issues.

I think I remember reading somewhere about someone doing what Grip has
suggested. He cut a foot or two from the middle of his kayak (or
canoe). I'll try and find it.