If you had no very specific knowledge about the new therapy, I would look at my budget (time, money, presonel), and find out how big a study I can perform gives these limitations.

If you have some idea about the desired precision of the effect size estimate you like to obtain, you can calulate the required sample size by relatively simple rules, depending on the kind of data you deal with.

For instance see

http://www.nrcse.washington.edu/NRCSE/struts/chapter2.pdf

If your model is more complex, including covariates and possibly reapeated measures or other covariance structures, you can run simulations to find out what a sufficient sample size would be.

Regarding the population size:

The concept of a statistical population is quite strange, rather metaphysical, and based on circular reasoning - so it is nothing that would be really understandable or helpful. However, this term is often used, and w.r.t. to this context the answer to your question "what well be your population size" is:

The population is the set of all elements (e.g. patients) that have a finite positive (and equal) probability to be sampled. So it depends how you sample, how you select patients.

If you know about the population at hand, you can determine sample size by means of:

Krejcie and Morgan (1970) sample size table.

If your are interesting in random sampling method you can also use the formula for sample size determination;

n = N/ (1+Nd2/10 000)

where:

N= Total population

d= error estimate with a confidence interval of 95%

n= sample size

You can also go for power analysis.

Respected madam, 

         since you are dealing with a innovative therapy, you can not apply it to everyone. you need to find the innovators who are ready to accept your innovative therapy and then you can proceed with your study. But if you want you do the survey on all the clients to whom you have tried your innovatibe therapy

dear madam

hope the attached  article will give a clearcut idea

Dear All RG members

You helped me and support me to solve this issue.I m lucky have you all

regards