It will depend on several things like how big it is when you get it, feeding, and growth. My 10 month old BP is 31 inches. Seems like he hit a major growth spurt in the last couple of months. You might consider starting with a 20L which should last around a year, I would think. Mine is in a 29gal. Which, I believe, is like a 20L floor space but a few inches taller...
Rob
Is the 29 gal what you are always gonna keep it in or are you gonna upgrade at all?