Should a 48-year-old person w/no stomach problems take an aspirin before taking a 13-hour flight (to prevent blood clots)? if yes, what dose?

Maybe: It would probably be safe to take an 81mg aspirin. However, it is more important during a long flight to ensure blood circulation by walking or otherwise exercising at least every few hours during the flight.

Depends: What kind of stomach problems do you have? Are you taking medication for your condition? If you have ulcers or stomach bleeding i would not advise even 81 mg aspirin. Best advice is to talk with your family physician who will give you exercises you can do in your seat with your legs, get up and walk the aisle periodically and wear support stockings.