It depends on what sort of life you want. I'm not a 'wheeler-dealer', so I'm keeping small. You need to go big to become a billionaire, but then it takes over your life.

You also need to think about what your product actually is, how many people want it, how much they want it, and how much they'll pay for it. Market size might make the big-or-small decision for you.

Finally, I think you need to consider what sort of problems your individual knowledge and experience gives you insight into. There are already thousands of PDF converters crowding search engine results. You need to find something that no one else has thought of (even if it's just a variation on someone else's product) so that there's something that makes your users want to give you money, rather than just using whichever competitor was top of the search results today.