I've read a bunch of people really like 4x2x2 but you could always trim some off of the height depending on how much you're snake climbs, if at all, etc. I'm personally in the process of building a 4x2x2 for a friend. His BP likes to climb for whatever reason so we're making it a little taller for extra branches, plants, etc.

So, I guess just triple that height if you wanna stack them all on top of eachother.