Dear Mohammad

The estimation of sample size depends mainly on standard deviation and the margin of error that accepted in the study, in the case of unavailable information about SD, then we can use the information from previous studies or the pilot study.

Please, read the attached file I hope to be useful for you.

Good Luck

You can always guess the standard deviation by asking someone who knows the area to tell you what would be an unexpectedly high and an unexpectedly low value. Divide this range by four. 

I got interested in this and used to ask, say, obstetricians what would be a high and a low fetal heart rate. They produced more or less a range of four SD. 

As previous answers indicate, the best thing is to go looking for any relevant information. Sample size calculations are always educated guesses (you haven't performed the experiment yet so you don't know the answer), you just want to find information that gives you some confidence in your guesstimate.

It is good practice to calculate for worst and best case scenarios as well as average situation, especially where uncertainty is high. If the difference is very large then keep looking for better estimates, if it is not then proceed based on available information.

For a descriptive study, although the sample size doesn't really matter, you should have enough numbers to prove that it represents the larger population using certain methods like standardization. For analytical study, if you are studying the whole cohort then calculate power. Or calculate sample size as mentioned above