closet poles are up to 16' long at the lumber yard. i don't know the max length of pvc. doesn't really matter because you can buy little devices to join the lengths and make it as long as you like.
fishing line? in the middle? i'm confused by the description of the way you're planning to hang it, so i hope this is helpful: run the pole all the way through the sleeve. hang it from the ends of the poles. it won't sag anywhere. if you' want to stabilize the bottom, too, then put another sleeve there and run a length of pvc. heck. you could put sleeves on all four sides and make a pvc frame for it. hang it from that. problem solved.