[deep thinker]

Not being a reel user as of yet I cant honestly say but I do know this much...
Your line to your spear is there for a few reasons, one of which is to retreive fish and enabeling you to go to the survace without letting go of your rig. The other is to be able to play fish should you shoot a big one that runs and also enabeling you to keep the fish off reefs or the bottom stuctures of the water you hunt seeing as alot of runners tend to head for the bottom and in this way run along the bottom in a effort to remove the spear and Ive seen this work on big yellow tail. So your line should be long enough to let the fish run but also enable you to keep the fish off the bottom so Id say line lenth would be determined by how deep you plan to hunt, so say your deepest hunt will be 20m youll have 25 or 30m line just incase you get a real big one that an drag you down...
But like I said I dont use a reel so maybe take my advice with a drop of salt water and see if someone else also replies