My software is a monthly subscription based service (cloud storage for photographers) but my market size is almost impossible to calculate. There are a reported 3.5 million photographers in the world but some speculate there can be up to 5x or more than that as most do photography part-time. Any suggestions on how to best present some top-down or bottom-up projections?
I took a quick look at 500px who reported 1.5-million users in 2012 (source: Techcrunch http://techcrunch.com/2012/11/28/gorgeous-photos-in-your-pocket-500px-arrives-on-iphone/) with 10% as professional photographers. The article also mentions monthly growth of ~100,000 users. If your targeting the same market as 500px, use their reported numbers to back-up your assumptions. I'm sure there are other services who target the same user community - I'd spend some time collecting this type of data.
Is your service designed only for professionals, or would you see a semi-professional (part-time) and advanced home-user?