You can make the sushi rice up to 4 hours ahead; cover and let stand at room temperature.
In a small bowl, stir rice vinegar, sugar, and salt until sugar and salt are dissolved.
In a fine strainer, rinse rice under cold running water until water runs clear. Drain for 5 to 10 minutes. Put rice in a 4- to 5-quart pan and add 5 1/4 cups water. Bring to a boil over high heat, then lower heat to maintain a simmer, cover, and cook until water is absorbed, 10 to 15 minutes.
Spread rice out in a shallow 12- by 16-inch baking pan and slowly pour vinegar mixture over it, turning rice gently and fanning it (the Japanese use a traditional fan, but you can use a piece of paper) until it comes to warm room temperature, about 15 minutes.