Just a matter of how many hours your pump will be running, what temperature you want to maintain, and if you're going to use a solar cover or not.
You have about 18,360 gallons of water, or 152,388 lbs.
To raise the water temperature 10 degrees, you need 1,523,880 btus (152,388 lbs x 10 degrees). Since your heater burns 266,000 btus/hr and with an average efficiency of 75%, you're only introducing 199,500 btus. Therefore, 1,523,880/199,500 = 7.6 hrs are needed of heater run time to get the temperature up an additional 10 degrees.
Here's the impact of the solar cover. If you do NOT use it, you should expect to lose approximately 50% of that heat, or about 4 hrs per day to maintain the 10 degrees increase after the initial heating run. If you use a solar cover, you'll lose approximately 20% of that heat, or a little less than 2 hrs to maintain it.
Most people will heat the pool on demand, rather than keeping it heated up. If that's the case, then expect for your heater to run at least 7.6 hrs in order to get your pool 10 degrees higher.
You can make the same calculations for higher temperature increases by replacing the 10 degrees in the first section above, with your desired temperature increase.
Regarding the gas line size, you're best having your local gas company or authorized pool contractor answer that.