[Floats]

Hardtimes,
The longer the bar the better, but the diff pumpkin gets in the way. Go back up to the sketch and see a longer bar sits at an angle in order to clear the diff.
As I see it, the longer the better, the bar should go from the chassis frame, all the way as far as possible and as close to the opposite wheel.
It should be as level and perpendicular to the frame and parallel to the axle.
However, this is not always possible and the Model A is not a racing car, and therefor a ‘half’ panhard will work just fine.
You could off course go for a full bar with a kink to clear the diff.
Regards
Chris