Question about sample size and margin of error

#1
I am trying to determine if my dataset sample size is large enough to be statistically significant. I'm using a 95% confidence interval. However, every formula I've found uses margin of error---and to calculate margin of error, I need a sample size. This seems circular, and I am lost.
 

Dason

Ambassador to the humans
#2
You can't talk about statistical significance without actually having data.

It sounds more like what you're interested in is doing a sample size calculation. Typically for these you specify a specific alternative hypothesis and a power you would like to achieve. Once you have that information you can solve for the sample size required.
 
#3
I do have a dataset, actually. I'm trying to determine if the samples it used were large enough.

I'm really new to all this, so let me back up a bit. I'm looking at a bunch of operating expenses for various properties, and I want to know if I can say "in x state, operating expenses were z on average" and have that be statistically significant. Not sure how hypothesis would work with that? (Also unfamiliar with power.)