I don't know if this will help any, but when I bought my bp, the guy said he would outgrow a 10 gal tank in 6 months (he was 2 month old when I got him) and then at 2 years he would outgrow the 20 long
tank.

I have a rough estimate of my bp's length, but I haven't gotten around to measuring his length yet. His current weight is about 110 g though.