Is the Audience Large Enough? Your audience will have to be big enough to sustain your business. It will likely also have to support competitors, as any successful business will attract competition.
Is the Audience Small Enough? Contrary to popular belief, I think that for a bootstrapped company, in particular, some markets can be too big. Some audiences are too generic, some industries too vast for a great niche to exist. At least, in the beginning, you should have a clearly defined niche. As a bootstrapped entrepreneur, you will reach out to customers and be in direct contact.