How sensitive are the decisions you are trying to make to the actual market size? A factor of two? A factor of ten?
Does the market exist? Does the need exist?
For startups, these last two questions are most useful in my experience: trying to attack a large market because it is well established is not normally a recipe for success unless you have some way to take advantage of a new trend or a new technology.
I would also focus much more on what's required for you to reach break even operation in a niche or segment since failure to do so will kill you more quickly than many other things.
Does the market exist? Does the need exist?
For startups, these last two questions are most useful in my experience: trying to attack a large market because it is well established is not normally a recipe for success unless you have some way to take advantage of a new trend or a new technology.
I would also focus much more on what's required for you to reach break even operation in a niche or segment since failure to do so will kill you more quickly than many other things.