Spaghetti is one of the most misjudged dishes in the home kitchen. Too little and someone goes hungry. Too much and you're eating pasta for three days. The good news is there are two reliable tricks for getting the amount right, and one genuinely good reason to make extra.
The standard serving of dry spaghetti is two ounces, which cooks down to roughly one cup. The challenge is that a pile of dry noodles is hard to eyeball. One classic method is the quarter trick: bundle the dry spaghetti and hold it upright, if the bundle fits through a circle made by your thumb and index finger about the size of a U.S. quarter, that's one serving.
Even easier: the opening of a standard 20-ounce soda or water bottle is almost exactly the right diameter for one serving. Stand the bottle on a flat surface, feed the dry spaghetti through the opening, and what fits is what you need. One bottle-opening per person at the table.
Spaghetti roughly doubles in volume when cooked, so two ounces dry becomes about four ounces on the plate.
Now, about those leftovers. Reheated spaghetti is fine, but a spaghetti frittata is genuinely better. Beat two or three eggs per cup of leftover pasta, mix in a handful of Parmesan and any herbs you have, and pour it into an oiled skillet. Cook on the stovetop until the edges set, then finish under the broiler for two or three minutes until the top is golden. It comes out like a savory cake, good for breakfast, lunch, or a light dinner.