It is a common misconception that a betta fish will be happy in a really small tank with just a few inches of water. While they do often reside in rice paddies that dry up and are only left with a few inches of water, it is not what they prefer. Rice paddies are usually fairly deep and are all connected to one big body of water. Betta fish do like their space, so a really small goldfish bowl will certainly not suffice.
For a single betta fish, the tank should be 2.5 gallons in size (about 9.5 liters) for the betta to be comfortable and happy as an absolute minimum. You should probably look into getting an even bigger tank of around 4 gallons (roughly 16 liters).
We have reviewed our Top 11 Betta Tanks in detail over at this article.
Betta fish do like plants, rocks, and driftwood, all of which require space. If you want to create a truly beautiful and comfortable habitat that feels like home, you need to add these plants and other items. To make sure that everything fits and that the betta has enough space to swim around freely, 4+ gallons is a good tank size.
If you have ever thought about making a Betta Pond then you might like this article, we cover how to make one the right way.
You should not worry too much about tank mates and their spatial requirements. While there are some fish that can coexist with betta fish, they are very few and far in between. Generally speaking, betta fish are best kept on their own.
The bottom line is that the absolute minimum tank size for a betta is 2.5 gallons being sufficient, and 4 gallons being the ideal. Remember folks, it is up to you to provide a nice and habitable home for your new betta fish!