I went ahead and purchased a Gast 0523 582.
Would I be wrong to use 3/8" line for the system outlined below?
The line coming from the barn will be 60 feet. After the initial run, it will be diverted into 2 separate runs with valves for each leg. The first will be 90 feet and the second will be about 200 feet with a diffuser at 100 feet and at 200 feet.
Like Bill said above, the short answer is yes. Long answer is the 3/8" line will freeze up a LOT quicker in winter, will cause higher back pressure in the system which will wear on the pump faster (which creates heat which increases the likelihood of the line freezing in the winter), and will provide a smaller volume of air to the pond than the larger airline.