You need to make sure you have enough people to market to in whatever area you decide to go after. These places will help to identify if you have a topic worth taking the time to fight after.
First you should make sure people are making money in the industry. One of the best ways to find this out is online. I suggest that you look at some tools to help you figure this out. This is going to give you a global view and might not personally apply to your local market if you are going offline, but it is a barometer to see if you are going in the right direction.
First thing I would do is check out the average big prices on Google Adwords free keyword tool. This will give you a good idea as far as how many people are searching for that keyword monthly. If you have a keyword that is over 100,000 searches then you are getting into a big niche and that is probably a good place to start.
Second check out the bids for those items on the average cost per click option on the side. This will give you a good idea as far as what companies are paying and how competitive it is. An average big will give you more of a realistic idea.
You want to see how many sponsored links are showing up for your keyword when you type it into Google Search on the top and side of the search results. The more the better for you because that means that more people see money opportunities in that field. If you are seeing at least 10 then that is a great sign.
Next I would go to Paypal shops and check out how many different books exist on your topic. If you have a whole row then that is good. If it is only a couple then I would look to expand into a bigger market first.
It is always easier to start big because you get a general flow of traffic and you can start to narrow down on specific niches. This will then allow for you to find the type of client you want and become successful with them. If you can do this then you will make a lot of money.
So check these steps out first and see what benefits they have to giving you a good idea of your market size. Then move forward to either shrinking or enlarging your pool.