Welcome to the forum
I'm not so sure there's nothing, but part of the forum is based on others sharing their experiences. I'm guessing, but pretty sure there isn't any active members who have done exactly what you are asking to respond.
I'm familiar with indoor soccer facilities, and I've seen everything from multimillion dollar complexes to bare bones facilities in a converted wharehouse/shop space. The actual size probably depends on how many fields you want. One in particular I'm thinking of has two fields and then a smaller one so teams can warm up and keep the games going.
I would go to many and participate as a customer, before considering owning one. I've had friends break bones playing, and I think there were probably releases where the player was responsible.
Regarding insurance, usually you will get more of a response if you ask one question per thread. I would ask that specific question alone in our forum for Insurance -
Again though, many might not have direct experiences so getting an insurance agent to advise you is probably the best route.
Hope to see you around more!