As important as length is weight. At 4" extension I bet a single spring on mine was 20 lbs. I couldn’t cut through a piece of 3/4" on top of a piece of 1/4" luan with the router all the way down in the mount. I took one spring off and it still wouldn’t extend far enough, and I’m not fond of pulling off side long term.
Buying locally was a crap shoot, no specs and not many choices. So I went to the McMaster site and found 2 that looked good and ordered them. I put both sets on, both worked fine moving the router manually. I left the second set on as it had a little more retract force, I cut the part I been waiting to make and it worked fine.
9654K195 This is one I left on. It’s a full loop end, I cut it to make it open loop but probably didn’t need to bother.
9433k76 A little weaker than the one above but nearly identical in appearance to the original that was on mine.
I also changed the belt, I figured it had been stretched too many times to have much life left.