As long as the shock doesn't top-out/bottom-out, then the length is acceptable. The damping characteristics are independent of the shock body length and/or diameter.
The majority of your problem is too stiff springs. The rule-of-thumb I use is to have the suspension compress 1/3 of total travel at operating weight.
At minimum, you need to know the vehicle's weight (front and rear) before you can find proper springs. You'll do better if you take the time to figure out the actual unsprung weight (wheels, tires, axles, etc.)