The bigger diameter cord you fit the less length cord you can use as the cord will be off the starter pulley so length will depend on the diameter of the cord.
Sometimes a bigger cord is used because of a greater breaking strength but more expensive thinner cord with a greater breaking strength could also be used.
I've used 3mm to 4 mm and don't worry about the length ,just cut the rope so the rope isn't too long for the pulley and so the rope isn't too long to over wind the recoil spring up when using a smaller rope.
Sounds like you are putting the rope on a difficult way.
When putting a spring in I would wind the spring in my hands then put the spring in one half of the starter ,(the housing or the pulley depends on the starter) then just slot the other half together.
The rope is easy ,unwind the starter spring (no disassembling) then use a lighter to melt the end of the rope then use fingers to squash the end smaller when hot but do this quickly so you don't burn your fingers.
Bend the metal tab up so you can wind the rope around the pulley to tighten the spring ,some starters have a notch for this.
Pics show unwinding the spring direction and tensioning the spring direction
Remember to bend the tab back down when you've finished tensioning the spring. Cheers Max..
