As a rule, 1 gallon per 1 inch of fish. So, Bettas should have atleast 1 gallon of water. And as others said, it will need a lid for a Betta as well.

If it doesn't come with a lid, I would buy a cheap plastic plate (check the dollar store) just a bit bigger than the bowl. Drill holes in it (for air) and lay it across the tank, bottom down.

It is very cute.