Large market size is important; but a startup invariably starts niche and then grows. When I try to explain this to entrepreneurs, I find that most people walking down the path of entrepreneurship find it very hard to understand the need to start niche. Everyone can use this product is the usual answer I get. Most entrepreneurs I come across, who are working on tech products or online businesses expect a million customers to show up in 6 months!
Starting a business is a marathon, not a sprint. It will most likely take 10 years of your life. It is in very rare cases that someone gets a Billion dollar exit in 3 years.
The larger goals and the larger market is important; but when you are starting out, the here and now matters more. You probably do not have the money or the capabilities to serve the larger market to begin with. What is most important is to keep moving forward a step at a time.
When starting a business the first few customers are by far the most important and the most critical. They are important because they put their trust in your product/service and decide to patronise you, at the same time, these are the people who provide you some of the most critical feedback which help shape your product/service. Also, it is probably the only time in your startup journey that you can spend time listening to each customer and understanding the qualitative richness of what they are saying.
Since it is cricket season, I am going to use that example. You might want to be the best national player, but you cannot say, you will play directly at the national level. You have to try to excel at club level, then get into smaller regional teams, and progress thereon. The greater goal of reaching the pinnacle motivates you but nevertheless you focus on the here and now. You try to score the century in the small matches that you are a part of. Shine at the opportunities provided to you. Make people realise and say that this player has got real talent. This probably helps a talent scout spot you and move you up into a higher level of the game, where you should be able to excel again to move forward.
Just because your product is online, does not mean that the world is your canvas. Start with the immediate; the people that you can reach out easily. Convince them to use it; take feedback. Find people who love what you are doing. They may be few, but that is fine. They will be the people who will make others patronise you. Keep growing the circle of people who love your product. As it keeps growing bigger and bigger, you will have a real opportunity to get into the big leagues.
Rome was not built in a day, it was not built in week or months. It was built over decades. That, most often, is also the case with businesses. Very few of them ever are built in a very short period of time. Have patience and build it one brick at a time.