I am going to put two resources here that I think will be helpful (especially the second one) in your situation. The first is going to be the actual formula but I think the calculator resources I am going to provide secondly will be even better for you as you can manipulate some numbers on it to get a better determination of what you need.
First, the formula used (in research) to determine sample size is
Z = Z value (e.g. 1.96 for 95% confidence level)
Confidence Level/ Value for Z
90% / 1.645
95% / 1.960
99% / 2.575
99.9% / 3.29
You probably would want at least a 90% confidence level for your results.
p = percentage picking a choice, expressed as decimal
(.5 used for sample size needed)
c = confidence interval, expressed as decimal
(e.g., .04 = ±4)
Now, this can get really frustrating for someone who is not really into the "math" side of the formula.
In my own research, I often use the following site which allows you to enter the information (data) and it will give you a good estimate of the sample size you will needCLICK HERE
for the sample size calculator
The first box is the confidence level you want (gives options of the 90, 95, and 99). The second box is the confidence interval you want (you enter a whole number - I would use 4 as a good starting point). This then allows a +4/-4 interval on either side of the level in the sample size. Finally, you have to enter a population. It is impossible to determine a sample size without having a "population" that you are drawing from. For example, if looking at ALL high school students, you would need to estimate how many high school students there are and put in that amount. Then based on the level and interval, you would know how many high school students you would need to sample in order to reach the confidence level that is needed.
I hope this helps. If I can be of ANY further assistance, please click on "Reply to Expert"