Hello there, great question! I would recommend either the top down or bottom up approach. I found a post that does an excellent job explaining this. The caveat would be if you have such a niche industry that you cannot use comparable data, I recommend sending out multiple surveys where you explain your product or service to understand customer interest. You can utilize those percentages (they more people you can get the better) to apply to this method.
I hope that helps. Good luck!
Your question is extremely vague. Basic analysis is a start. You list no elements so other than answering by saying "Do your research" there is no way for any one to put any strategic thought into your question.
Take a look at this post I wrote about Market Size Assessment. There you'll find a spreadsheet designed to help on numbers calculations.Please, let me know if you need any help.