It is virtually impossible to measure the size of the virtual market. The number of people joining Android on a daily basis is increasing but then along comes WP7 and that may change the rate of growth. Then you have out of those people on Android the percent who share an interest in what you are developing... but the only way to reach those people is by identifying who and where they are. In-app advertisements and forum postings have been mentioned but then even only the select few who may be interested in your subject will click/participate. Yes, you face a virtual impossibility.

In our start-up world you simply cannot predict success on the measuring the size of the market. It's impossible. Think back to 2007 and tell me: What is the size of the market for Fart apps? And it will cost $0.99? My guess? Very, very limited. Huh. Guess I missed the mark on that one. Now, think back to 2005. Tell me, what is the size of the market for people to blog in 140 characters or less? My guess, pretty small. I mean, Facebook can do that. Oops. Missed that one, too. OK, now that Twitter is HUGE what is the size of the market for people willing to pay for it? A ha! Studies show near zero.

So the size of the market willing to pay $0.99 for fart apps is far bigger than that of Twitter. Amazing, eh?

So here's what you do: Go build an MVP, identify some leading influencers who could be interested in it, beta test it out, and gather feedback. That's all you really can do.