Snowboard length for 5'9" boarder?

I'm 5'9'' .. been growing a lot lately and likely to keep growing. I'm looking at a Forum Substance .. trying to order it through the-house.com but they only have a 152 .. I'm thinking that may be too small and I'll outgrow it .. I also like the Forum Manual and they have 152, 154, 156 ... so there's a lot more choice and I'm looking at getting the 156 ... should I wait until they get more sizes for the substance or should I just go for the Manual .. because now is the time to buy boards cheap and if I wait .. prices may go up.