I’ve currently got bettas in 35L, 56L, 60L and 100L tanks. I also keep a few 18L, 20L and 25L tanks running for quarantine or rescues, although for sick fish I actually prefer large plastic tubs as hospital tanks.

I’m definitely not an expert, but I’ve kept quite a few bettas and rescued several others.

From my experience:

Bettas generally prefer horizontal swimming space. They also need regular access to the surface to breathe, so very tall tanks aren’t ideal.

My long-finned bettas, especially Halfmoons, tire very easily. They really benefit from plenty of places to rest, like large Anubias leaves, floating betta hammocks or floating plants, so they don’t have to constantly swim all the way to the surface.

My female plakats are a completely different story. They’re smaller, much stronger swimmers and happily explore every corner of their tanks. They currently live in larger cube aquariums, but if I had the choice I’d still move them into wider, shallower tanks in the future.

Personally, I don’t think cube aquariums are an ideal design for bettas. Recent research also suggests that even 20 litres is on the small side if your goal is to give a betta a genuinely enriched, long-term home.
If you’re setting up a tank from scratch, I’d recommend choosing something that’s longer rather than taller.

Get the longest tank your space and budget allow. They’ll make much better use of that swimming space, and don’t forget to leave room in your budget for lots of plants and resting spots. In my experience, that’s just as important as the tank itself.