Subtrate size?

[Singingcrow]

Hello!

I have been looking and looking for and answer to this but haven't found what I'm looking for, so I hope you can help me, and if there is a thread on this and I was blind, please forgive me for the repeat.

What I want to do is mount my phal noid to a chunk o wood that fell out of a tree onto my balcony in the summer. However, I'm not sure if it is big enough in diameter. I know I'm supposed to think about how big the plant will grow, but that's a problem, because this is my first phal and I don't know how big they can get! Is there a special formula like there is with potting, i.e. so many inches larger than the root system???

Please help! I'm on the verge of buying another phal and I think I have to get this one in order before I do. But I don't know how long I can control myself!!!