2 Sample T-Test (Sample Size)

I'm trying to calculate a required sample size, but have a feeling I don't have enough information.

Situation: Two Randomized Groups: 1 no intervention, 1 with intervention.  If there is a 10% or greater increase in size, the intervention has made a difference.

I believe that is a 2-Sample t Test (1 tailed). I assume Power = .8, but since there hasn't been sampling, I don't know SD.

If someone has a moment, can you let me know what I'm missing... it just isn't computing (literally).

Cheers,

M

PS: I accidentally posted this in Healthcare too.... sorry about that...