Assuming your are within the weight limit, typically 240 lbs, your individual weight relative to those flying with you, will determine your seating position. Most often the smallest person on the flight will be seated center front in the “co-pilot” seat with their partner next to them.
The two largest people left will be in the center back with their respective partners next to them. Occasionally they may have to separate couples for safety reasons.
For persons who exceed the weight limitation, you will have to pay either a “comfort seat” fee or purchase a second seat, depending on the vendor. You would most likely be seated in the center back.
Interestingly, this formula may change due to the size of the pilot!