10 gallon if you have the room, but a 5 is fine if he's going to be the only occupant
I have mine in 5 gallon cubes & they're doing fine
P&H have cheap 5 gallon cubes, always get the biggest you can afford & have the room for, your Betta will be happier with more space
Imo 7.6 litres is too small, that's only 2 gallons & by the time you've added substrate etc, would probably only end up being 1.5