I'd say a minimum of 20 litres, but if you want a really happy betta fish try for around 35+ litres. The actual footprint of the tank won't be much bigger than a smaller one, but the larger volume of water will be easier to maintain, better water quality, any water issues will take longer to become a problem as it's more diluted, easier to have a filter in there as it doesn't take up so much of the room and can lessen the effects of the current (bettas are tired out easily), and there's also more room for decor such as a plant and a hollow ornament for the betta to hide in.
Having kept a number of bettas over the years in everything ranging from 5 litres (in an emergency) to 35, I know which size they have preferred.