a lot of people will say a 20g, but you're talking about a snake that can get over 5ft long. CA state laws requires that a snake be in a cage with a perimeter no less than 80% of the total length of the snake(don't quote me, i can't find a quote to back it up but basically a 4' snake could legally be kept in a standard 10g) but if you've ever seen a snake in a tank that small, they look really cramped and can develop lung issues. why not just give the snake the benefit of a little more room and go w/ a 40g.