You can very easily get the 14 pound fireline. It has a 4 lb diameter, if I am correct. The 20 pound fireline has a 6 pound diameter. If you want something for your ultralight rod, I would say go with the smallest fireline that you can find. I think they make 10 pound test that is 2 pound diameter. I have used the 14 lb test on my ultralight and loved it. I gave up a couple feet in casting distance, but that was all I gave up. I was able to pull lures out from under rocks, heck, I broke hooks before the line would break and that is no joke.
Here is another tip, if you get fireline, put it on the reel, then take your fishing rod outside and cast for awhile. The line is kinda stiff when it is first put on the reel, after awhile, it loosens up and it casts about the same as monofilament. O yeah, it has NO memory too, so you don't have to worry about those pesky curls in the line.
Good Luck This Season: Buck Magnet
"Hunting is not a sport, it is a passion, it is a WAY OF LIFE"