There are 4 inputs needed to estimate sample size for a test: power, confidence level, expected difference, and variance. You need all 4 before you run any test. You use the A/A test to estimate variance. Power is the probability of detecting an x% difference when one really exists. Typically you see .8 or .9. Confidence level is the probability of detecting a difference when one really does not exist, typically .05. The 4th item is the expected difference of the test. If you want to detect a 1% difference, you will need more sample than if you want to detect a 5% difference.

You have to know all 4 before you do a test. A test is designed specifically to detect a certain difference. You cannot launch a test without knowing that as part of your hypothesis.