https://www.youtube.com/watch?v=cCP0vkCjmQQ&feature=youtu.be

I know this is an older video, but in case for anyone who every comes across this, clearly by the like:dislike ratio and comments, that this video is highly informational. I'm gonna give a more detailed reasoning as to why I agree.

Whether you believe 1g, 2.5g, 5g or 10g is the betta minimum, let's consider the space each one actually provides.

1g (4 liters): 4.6 x 9.1 x 9.9 inches | 12 x 23 x 25 centimeters
2.5g (9 liters): 12 x 6 x 8 inches | 30 x 15 x 20 centimeters
5g (19 liters): 16 x 8 x 10 inches | 41 x 20 x 25 centimeters
10g (38 liters): 20 x 10 x 12 inches | 51 x 25 x 30 centimeters

Now, for me, I view 5g the bare minimum for bettas (specially Betta splendens as any other betta specie should require a larger tank). Now, I have a few reasons I view this. Before I explain, everyone is entitled to their own opinions and of course minimums are meant to give the guideline of the most basic care available for the animal, but I find that we, as general animal keepers, to strive for minimums that allow for the animal to thrive rather than survive/just get by.

Sources:
Note: any links to a product are not recommendations. They are are to cite where I got information (like the 1g betta tank).
Note: Conversions of gallons to liters and inches to centimeters were done through Google searches as well as being rounded.
https://www.amazon.com/Tetra-Half-Betta-Aquarium-29049/dp/B007GCGUUY/ref=sr_1_2?keywords=1g+betta+tank&qid=1564804362&s=gateway&sr=8-2
http://www.aquariumdimensions.com/