The question “what is your market size” sounds hard to answer for a small business.
It need not be, someone has probably done the work for you, its just a case of where to find it.
With my marine product range, http://www.rigtube.co.uk and http://www.hikingbench.com I sell to the dinghy racing sailor. I know there are a lot of sailing clubs around the country, but how many?
I looked on the Yachts and Yachting magazine (online version) and found a directory of sailing clubs together with a map of there locations. Click to view map:
http://www.rigtube.co.uk/images/Sailingclubs.jpg
Its likely, if you hunt in specialist magazines for your market sector you may well find something similar.
This is because a number of publishing groups publish most of the magazines. Market sector is very important to them and staff move between titles, so the framework used will be similar.