How can I figure out the market size for a mobile app (and then calculate how much money I'd make)?

I have an idea for a VoIP mobile app on Android that would provide some advanced technical features not currently available (i.e. better encryption, firewall traversal, etc.) The problem I have is no idea how to monetize this idea, or whether there is significant demand for this kind of thing.

What can I do to make an educated estimate on the size of the market for this product and calculate my expected revenue?

I too was experiencing a similar problem a few months ago.

Here are a few tips:

1. Understand the competitors. Who are they and what sort of revenue model are they implementing? Is their revenue model successful? Because that particular question is hard to answer consider assessing whether or not the revenue model is palatable to users (i.e. ads are not disruptive), the number of downloads, number of users who update, etc. This will not give you revenue data but it can be correlated back to your revenue assumptions.

Nice article summarizing the different types of app revenue models: Monetizing Mobile Apps

1. Your market size. This is information I had a very difficult time obtaining. One website that did estimate the number of users for different Android apps is AndroLib. You may also want to find a company (e.g. Zynga) that only develops applications as company products and see if you can go through their financial reports for more information. However, do keep in mind that many of these types of companies are private and may not share this information.

Hope this helps. Best of luck!