Sample size calculation

I need to study the outcome of an intervention in a disease. I will be doing a prospective observational (longitudinal) study in a single cohort (I am not sure if i'm correct - I mean, I will be taking only a single group of diseased undergoing the proposed intervention). The prevalence of the disease is ~2.6%. The mean +/- SD of the measured outcome in previous study is 253 +/- 9.
So, How will I proceed further with the sample size calculation?
Any help will be greatly appreciated.
Your sample size will be just the number of diseased, randomized to treatment or not? So the prevalence thing is sort of irrelevant except insofar as accrual is concerned. Is it H0: intervention does not effect outcome in diseased dudes (dudettes)?
I want to see what is the outcome (measured by a scoring system) of a surgery in a certain fracture. I will only be taking the treated ones (no comparison).
ok so you just want to estimate the mean score in on e group of fractured people. well then you have said the mean sd is 253+-9? Is that true or up for dispute?