You have to measure out the rectangle in your yard. 16 feet wide x 42 feet long.
You can buy a testing kit and test your water to see if there are any abnormal bacteria in your water that would be keeping the chlorine levels low. Otherwise, just add chlorine daily to keep your pool safe for swimming.
Depends how deep it is! 16 x 42 = 672 square feet 7.48 (us)gallons to 1 cubic feet 672 x 7.48 = 5026.5 gallons for every 1 foot of depth (eg 4ft deep = 20106 gallons)