Rod lengths were selected by casting that I have to do. The longer the cast, the longer the rod.
Line weight is determined by the fly weights that I am casting. Drys were always 4, nymphs 5, and streamers 6 or 7 weight lines. Bass bugs were 8.
I have slow action rods and fast action rods, each have their own feel for what I want to do. But the slower actions like fiberglass, bamboo, and early graphites were what I learned with. Faster actions have a goofy feel, but can be cast well. Just not by my learned arm, which is now slower since they took a chunk out of it to make my tongue.