[Versión en español]
So stiffer rods require heavier lines to flex them. The question is, how does one decide what is suitable for you? The answer is in the fly!
Small flies require thin tippets (the bit of leader material which attaches the fly to the rest of the leader). Big flies, on the otherhand, require both thick tippets and a heavy mass of flyline to carry them out there. Small flies are great for small streams. Large flies are great for the salt.
As a general guide I'll use a 4-weight for river fishing, a 6-weight as a general all-round trout fishing outfit, an 8-weight for general saltwater use and 10's and upwards for heavy saltwater / salmon fishing.
And so what of rod lengths?
I use 9ft rods. The ultimate length for distance casting is 9ft 3 inches (I read somewhere) but this depends on technique and build. Longer rods allow greater line control after the line has landed. Shorter rods are superb for casting in tight overgrown places.
A good starting rod is a 9ft rod for either a 5 weight line (if you are mainly river fishing) or a 6 weight for general use. For reservoir bank fishing a 9 foot 6 inch AFTM 7 is standard.
If you are going to fish flies the size of turkeys you are going to need an AFTM15 and a broomstick