It dosen't do any harm to have say 200yds of backing on a reel what so ever at all, especially on bigger rivers. Increases the retrieve rate of your reels for one thing. and helps prevent any line from becoming coil bound too. I'd agree in that most fish will rarely if ever run out a full 40yd line, or a head and full running line, but when you need it that 1st time, you'll soon understand why it's a sensible precaution. In a big pool or pool tail out, all it takes is for Salar to stop swimming and drift off with the current or use the flow and you can loose 100yds in the blink of an eye!.
As far as diameter of backing go's, well if its a large capacity reel I find its better to fit backing in two stages.Your normal run of the mill 30lb dacron next to your line and then the thicker stuff to bulk up next to the reel spindle.That way when you do have a fish run you into the backing, the level and thus retrieve rate won't drop alarmingly, you'll still keep a reasonable diameter to the reel for retrieve and very importantly to slow the speed the reel revolves at!.
Some of my Marquis 3's carry in excess of 300yds of dacron.If that ever see's the light of day, any fish is about to drop off or break me pretty soon!.
Pedro.