For a standard side dish portion, the general culinary rule of thumb is between 1/4 cup and 1/3 cup of dry rice per person. In terms of weight, this is approximately 50 to 65 grams (or about 1.75 to 2.3 ounces).
White rice generally absorbs water and triples in volume as it cooks. Therefore, measuring out 1/4 cup of dry uncooked grains will yield roughly 3/4 cup of cooked rice. This makes a very satisfying side portion alongside a protein and vegetables without leaving unwanted leftovers. If you are quite hungry or the rest of the meal is relatively light, measuring out 1/3 cup dry will yield close to a full cup cooked.
To prepare a single portion quickly, use a 1:2 ratio of rice to water for standard long-grain white rice (for example, 1/4 cup of dry rice to 1/2 cup of water). Bring it to a gentle boil, cover tightly, reduce the heat to low for about 15 minutes, and let it rest off the heat covered for 5 minutes before fluffing with a fork.