For a standard evening main meal, the general rule of thumb is between 75 grams and 100 grams (about 2.5 to 3.5 ounces) of dry pasta per adult.
If you are preparing a dish with a very rich, heavy sauce containing meat, cream, or plenty of vegetables, 75 to 80 grams is usually plenty to leave you satisfied without having leftovers. However, if the pasta itself is the star of the dish—such as with a light tomato sauce, aglio e olio, or simple pesto—aim for around 100 grams per person.
For quick measuring without a kitchen scale, a 100-gram portion of long pasta like spaghetti is roughly the diameter of a standard one-pound coin (or about 2.5 centimeters across) when bundled together. For short shapes like penne or fusilli, roughly two level handfuls per person will get you very close to an ideal portion.