There is no a minimum number of observations that are required in order to have a balanced panel. There is no a rule about that... Generally, regarding the number of observations the greater the better. So your data are fine for me in order to construct a balanced panel. 

Dear Ali,

A "balanced" panel data-set only means  that you have the same amount of observations per panel, it has nothing to do with the size of the data set:

https://en.wikipedia.org/wiki/Panel_data#Example

But I agree with Dimitrios, this seems to be a big enough set for analysis. However, you can't use fixed effects with this data, because you would use a sample of n=4 to estimate the FEs. If you want to control for fixed effects use demeaned POLS instead.

Hope that helps,

Tim

Dear Md Ali

The minimum sample size depends on the type of your population, is it finite population or infinite population.

In the case of finite population, you can use the attached table to estimate the suitable sample size.

But in case of infinite population: 

The sample size for any study depends on the standard deviation of the variable ( from previous studies ) and the margin of error you decided . My advice to use G*Power .

G*Power software is effective tool to calculate sample size for many ranges of experiments. Also, you can determine effect size and power of the test, G*Power is free to download and easy to use after reading the manual, the download link:

http://download.cnet.com/G-Power/3000-2054_4-10647044.html

The link to download G*Power manual:

http://www.gpower.hhu.de/fileadmin/redaktion/Fakultaeten/Mathematisch-Naturwissenschaftliche_Fakultaet/Psychologie/AAP/gpower/GPowerManual.pdf

Good Luck

Thank you, Dimitrios, Tim and Khalid for your answers. Now, regarding the sample size for my study, I would say 55 firms have been chosen out of 680 listed companies since they meet certain criteria for selection. And these 680 companies are financial companies. Because of the particular criteria, I have used a purposive sampling strategy (a non-probability sampling technique). Now the question is, do you think this is an appropriate sampling technique for generalising the results to the entire population? Can I use it for my study?  Also, do you think 55 selected companies are well representative of the whole population?

I need your suggestions, please.

Regards,

Md. Ali Rasel

Thank you all for your input.

Regards

Andrew Marks