http://www.steffysgarage.com/downloa...line-chart.pdf
Too large of lowering block will make the trailing arm drag the ground in the event you would have a flat on that side.
(basically holding the wheel off of the ground)
This would affect your ability to stop using that wheel's brakes.
It also makes the trailing arm very close to the ground.
I never use more than a 1.5" lowering bock with 15" wheel.
On my trucks with 20" wheels, I use a 2" lowering block.