You can best establish the area wanted for a window, yourself. When you are trapezing, look in front of the mast where you can see what you would like to see if the mainsail was not blocking the view. Same thing to the rear of the main. Look across the sail to establish an area on the main that matches or connects what you see in front and behind. Now that you have established the area in the main that you would be able to look through, if it was transparent, to see what you need to see, then take it to a sail maker and ask how much of that area can be made into a window and still maintain the structural integrity of the sail. I think you will be surprised at how large a window can be installed. Another, uncommon, way to do this is just to put as large a window as you want in the sail and then add reinforcement as needed. I have used both these methods with excellent results. There is nothing like a large window to be safer and better enjoy any sail. One other note: there are different types of window material which can make a big difference - one type has a diamond grid of reinforcing fibers and the other is just clear, heavy mylar. The first type is not as easy to see through but can make a very large window easy to do and the other is very clear but will require some material in between to make a very large window in several panels. Your sail maker may even have some other options to offer. If you do not have a local sail maker, check with several about this until you find what you are looking for in terms of results and service and price. If you would like to see some sails that are essentially all window, then look at the Tasar website for the new sails they are using.
