Drain pipe size is based upon watershed acreage, annual rainfall, (IMO how heavily the rainfall is concentrated or spread out over the year), and other (lesser) facts like available space above the pipe before the dam would over run. Local NRCS office is a great place to have this calculated if they are cooperative (reportedly some are, some aren't). Texans near you could help you figure it out the best, but will need your watershed size.

"Live like you'll die tomorrow, but manage your grass like you'll live forever."
-S. M. Stirling
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