There's pea puffers for freshwater, I've kept one before in a 10 gallon divided tank. Male Betta being on the other side of the tank. probably 10 gallon for a pair would suffice.
My favourite freshwater puffer right now, which I might get again one day is the Irrubesco Puffer. I'd say 10 gallon for one and 15 gallon for a pair.
I'd recommend strong filtration, since theyre quite messy n poop quite a bit. I've tried some tankmates with them but theyre best kept in a species tank. Puffers are very nippy, and will nip at tankmates to death. The one fish that I remember never got nipped at and held his own well, even chase the puffer before it even gets too close was the Siamese algae eater. Theyre active day n night and extremely fast. I still suggest to keep them species only.
My old Dwarf/Pea Puffer
Irrubesco Puffer from one of my subsriptions on youtube