"Which size am I?" The ultimate dilemma. Should I go for the smaller or larger reserve? This is especially difficult to decide when you are on the borderline between two sizes. On one side - you have a lighter, smaller package - with slightly faster opening times, better pendulum stability, but also higher sink rate - and on the other, a slightly heavier and larger package, but with excellent sink rate. In the end actually - it's all a matter of personal preference and flying style.
Let's define some terms commonly used:
Max. load - usually the maximum takeoff load the reserve has been tested for, and therefore is the maximum permitted TOW (takeoff weight) it should be flown with. This is the loading under which the rescue has passed the drop testing;
Recommended load - This is the weight range which is optimal by the manufacturer. In this range the rescue has the best sink rate/stability ratio.
Load/Takeoff weight - The declared maximum and recommended loads always take into account your total take-off weight,including your weight, your wings weight and the rest of your equipment. The reason the wing weight should also be taken into account is because your wing weight can also have a slight affect your descent depending on multiple factors:
1) Whether you jettison your wing or not (quick out carabiners etc.);
2) Roll your wing into your lap;
3) Keep your wing inflated;
Size selection mainly depends on your flying style and preference. If you are borderline between two sizes, you have two options:
1) If you are a recreational pilot in need of a primary rescue, with a TOW around the optimal range of the larger reserve, and are prepared to "sacrifice" a bit of weight, but in return get a lower (safer) sink rate, it is always wiser and safer to go for the larger one.
2) If you want to go for the lightest setup possible (for instance for hike and fly), or you plan to use the reserve as a secondary reserve, and are reasonably fit - you can go for the smaller reserve, but have in mind the higher sink rate accompanied with this decision.