I'd guess you're more likely to fail building a product for 'everyone' than for a niche. Therefore I'm not sure it's productive to worry about the size of niches unless you really are only interested in building a huge company (which seems not to be how most people who actually do build huge companies start out).

If you're aiming for a niche you likely know something about it - or at least can learn & validate quickly. If you're aiming for 'everyone' you're likely either a) solving a non-problem that everyone has (e.g walking: Segway) b) solving a problem nobody has (it's too small, or non-existent).