Does anyone have these in their ponds? I was looking online for some basic info, but it's kind of sketchy. Stuff like overwintering them, their adult size, etc.

I've never seen one or read much about them. They are rare in the US. I assume you're in the UK.

No, I'm in Canada.
I was just curious as they sound like a very interesting fish from what I have been able to find out, but they do seem to be mostly in the UK for sure, although I did find a site that said there were a few lake in western US and Canada that they were introduced to and are serviving in.