I just did a 25 litre brew in a 30 litre fermenter with MJ M42. Glad I caught it in time before it exploded, spent the next 12 hours watching it vent foam out of the bung hole. You can't be too careful. But I think if I'd had another 5 litres headspace, the result would have not been much different.
Bit pricey for a plastic fermenter.I have 10l glass fermenting jars from fleabay which are around £14 but are wide mouth and great for smaller batches.
Personally I would spend a few extra on a glass carboy but they are heavy with a very narrow neck.
I have 3 of these. The handles at the top are bendy so you can carry them full. Even one handed if you are strong. Very clear plastic and the neck is just wide enough to get an arm in, so cleaning is easy. A small amount of sediment can settle on the ridges, but a sharp twist of the fermenter releases it. I like mine but they don’t fit in my new fridge so I mainly use for bottling day lately. If you put anything hot in them, they distort permanently. Under 60c is fine.
30 litre capacity Blue plastic barrels with red screw top lids. 2 side handles for easy lifting. Washed and ready for reuse . No chemicals used. Ex food stuff use only. Used once previously. Ideal for storage of pet food etc . Very good condition for reuse. 75 available