In the last year fly fish line manufactures have gotten completely out of control There is now a specialty line for nearly every situation and species. Some of the more ridiculously specialized lines are a shooting head specifically for great lakes tributaries, a hover lake line for fishing just below the surface, a even a specific line for Carp. You can get wind cutter or streamer and warm water line. It never ends. You can even buy a line made to replicate old fashion silk lines. The casting technology of the 1800's is BACK! YES!
While we are at it I have a few suggestions for the line manufactures
Double-tip 3000. Sinking and floating line in one! Just switch form left to right handed retrieve that the line floats!
Water slapin-hopper line. Extra wide and flat tip section that splashes down like a cannonball. Finally you can splash down your line like you mean it and get the attention of those BIG fish!
Race for the cure pink line ( I seriously think it is in development) Real men cast whatever color line they want.
Season 3 fly line. Why wait three years for that sinking tip and guide abrasive feel? This line feels three years old right out of the box!
Muskie line This line has a kevlar welded tip with 10,000 pound break point for chuckin some serious hair and feathers! No more busted loops. The line is 200' long for those super long casts and comes with a beer bottle opener that replaces your fighting butt (I already have one!). Best of all, it only comes in BLAZE ORANGE!
Okay, I'm getting carried away. I used to think rods and reels were getting out of hand for cost but actually, I think I spend (and in the future will spend) more on line than anything else. Here is my best breakdown of line choices
There are essentially two types of tapers double taper and the more popular weight forward. Each manufacturer has a specific length of each section and taper between those sections depending on what the line is used for or the wt of the line.
Weight forward lines have a sections that start thin, get thicker (~30') then get thin again. Once the thicker (and heavier) section gets out of your guides and in the air it helps you cast farther. Also, the second thin part (running line) shoots faster through your guides than a thicker line would (less friction).
Double taper lines start thin, taper to thicker for almost the length of the line then taper the same way at the other end. So, they are reversible and, therefore, last twice as long. They are good for delicate close range presentations and excellent for mending.
Trout: 1-5wt rods. One reel one spool.
I don't over line these size rods and I use weight forward (WF) for my 5wt and double taper (DT) for 3 and down. I don't have a 4 wt but I would go for WF line there. My logic: You don't cast 3wts and smaller very far or fish them in the wind. They excel where DT line excels. Easy lay downs and delicate presentations. I fish weighted flies and cast farther with my 5 wt so I like a WF line. If you have to mend a lot on the river you fish go with a DT. The thinker belly section mends a lot easier. "Trout" series lines just mean the taper has been designed to lay down easy. What they don't say is just get a double taper because it will last twice as long! WF lines are for casting farther. DT lines are for soft presentations and better close range casting. Just pick one of those two styles.
6-9 wts One reel two spools
I would get two spools with any bigger reel you have. These would fish streamers and larger flies well. A floating line and a sinking line would be good for different situations. I have a super fast action 7 wt and I over lined it with an 8 wt line so I can feel the loading of the rod a little easier. Sort of like training wheels. GPX line is just a half size up from the size on the box. Most streamer lines, wind cutter lines and bass lines are probably a half size or more up too. You can just get a normal line one size bigger instead of a fancy named line for every species and condition.
One real difference between line in this category is whether they are for warm or cold water. Cold water lines will stay supple and cast well in the cold. They will also turn into jelly in the tropical heat. Warm water lines will stay the right consistency in the heat but be like casting cement in the cold (by cold I mean below freezing). I don't have a specialized warm/cold line and I haven't noticed that my line sucks in any condition. Unless you only fish in a certain climate I would stick with a line designed in the middle of the two extremes.
Bigger? I have no idea but I would guess casting a muskie or tuna fly on a 10 or 11 weight would benefit from a line size up. Maybe not.
Casting aficionados also posit that each rod has a "perfect line" and they will try out 10-20 lines of the same weight for their rod to find their "match." I'm not that good of a caster to notice. I have cast everything form $20 fly line to $99 sharkskin. My favorites are Scientific anglers mastery series or the Rio gold line. I don't like textured lines. They shoot like a rocket but I hate the noise they make going through the guides and they pick up dirt like crazy. Plus, why exactly are they a hundred bucks? I thought $70 was out of control.