This recipe only uses half of your spaghetti squash. If you're able to find a small squash (2 pounds-ish), feel free to use it, keeping in mind that you may need to shorten the cook time. Alternatively, wrap up your spare squash half and store for later. It will keep for 2-3 days in the refrigerator.
Feel free to heat the sauce in the microwave, or not at all, if that's your thing.