Ch4rlie said:
So, the new question is likely best described as: "What is the minimum optimum size tank for a LONE betta?
This is probably the main debate that Tcamos is asking for.
As per previous answers given already, there is a range of 'optimum' tank sizes that these folks would personally set up.
However, I like this statement from our own member, Wildbetta, she has written an very good article on care for betta splendens for this forum, and as she is a very experienced betta keeper and breeds rare species.
Size: The minimum size tank that a single betta can be kept in is 2.5 gallon. 5 gallon tanks and above are easy to heat, need to be cleaned much less often, and result in a happier and healthier betta. Contrary to popular belief, there is no maximum, and bettas do not like small spaces. Bettas kept in larger tanks are healthier and less prone to problems such as ammonia burn, bacterial infections, and obesity.
Betta Splendens Caresheet
I think that is a good starting point for finding what are 'optimum' tank sizes for a lone betta.
Yup, I've read that plenty of times, but this is the science section, and as much as I agree that WildBetta is highly regarded and experienced, I don't believe that her opinion by itself is 'authoritative' enough to completely end the debate.