There's not really a minimum batch size as long as you've got enough line on the floating dip tube to get the beer out.
How big is your average batch size?
You'll see people offering opinions that too much headspace damages the beer. I think the reasoning goes that with all that headspace you'll lose more of the esters and hop aroma than if you fermented in a smaller chamber. Sounds plausible, and there's a bit of evidence from a few brulosophy experiments. But there's a lot going on in those experiments so it could be other factors.
It should be less of a problem in a fermenter that can take pressure, though if it's real it would still be happening. And if you plan on serving from the fermenter it'd be even less of a problem. It sounds like it'd be better to avoid too much headspace if you can, but I'm thinking the lower level of oxidation that comes from the system may make up for the bigger headspace.
Though that may be me justifying things to myself as I plan on ordering the 30 litre version myself in the next few days. That will have a decent amount of headspace as I'm generally around a 13 litre brewer. So maybe I'll jump to the other side of the highway and get a smaller version -
https://www.keg-king.com.au/fermenter-king-junior.html