Hi all, i recently lowered my 1989 Volvo 740 GL with BC coilovers and was told I should get an adjustable panhard bar aswell. How much different in length should i make it from the stock one?

 

It depends on how much you lowered the car. The axle will shift to the right (IIRC) as the geometry changes. The adjustable bar can be shortened, causing the axle to return to a neutral, centered position.
This is often easily determined by putting the car on jack stands, adjusting the bar, stepping back and looking at the wheels and body to see if they align, and repeating until it looks correct.