Well, the weight to uphold stays the same, no matter the strap length.
What does matter is;
1. the angle the strap makes on your shoulder for this changes the the amount of body contact surface
2. the width of the strap for this changes the amount of body contact surface
3. the position of the strap on your shoulder
4. the shape of your body
5. the physical condition of your muscles
The main muscle that need to uphold guitar through the strap is the trapezius.
The function of this muscle is to lift your shoulder and to position/stabilize your shoulder blade (it's predominantly a posture muscle).
This muscle is quite large and has several sections. The upper section goes from head & upper vertebrae to top side of the shoulder blade & clavicle.
This part is where the strap rests on also.
The further the strap is positioned towards the shoulder the more work this muscle has to do. So it would be better to have the strap rest more towards the neck.
The problem is that there are many variations in trapezius angle in humans. If you have a sharper angled trapezius (which also enhances when you do fitness work out) the strap will automatically slide more towards the shoulder.
When you make your strap too short you will have to lift your arm sideways a bit to reach the upper frets, which in turn will activate the trapezius. So a high position will fatigue the trapezius earlier.
When you make the strap too long, you get the feeling it will slide off of your shoulder, so that activates the trapezius too (and will cause overuse of your wrist because of the increased angle).
Training of the trapezius, like dumbell flys and shoulder press will help you to have less fatigue when playing standing up.
