Measuring stretch forces isn't easy - unless you have some conductive rubber cord! This cord is 2mm diameter, and 1 meter long, made of carbon-black impregnated rubber.
In a 'relaxed' state, the resistance is about 350 ohms per inch. As you pull on it, the resistance increases (the particles get further apart). As you stretch it out, the resistance increases. So lets say you have a 6" piece - thats about 2.1 Kohms. Stretch it to 10" long and now it is 10"/6"*2.1K = 3.5 Kohms. You can stretch the rubber about 50-70% longer than the resting length, so a 6" piece shouldn't be stretched more than 10". Once the force is released, the rubber will shrink back, although its not very 'fast' and it takes a minute or two to revert to its original length.
Length: ~1 meter
Resistance: 350-400 ohm/inç or 140-160 ohm/cm