Line weight determines the size of load the rod will carry. A 4wt. rod that is 9 ft. long is suited for small flies, whereas a 8 wt. rod in the same length would be adequate for tossing large flies for bass, salmon/steelhead, etc.
The 9’ rods will have 10 or 11 guides, including tip. Fly fishing is a bit different than conventional, in that the line weight is the primary element, rather than the length.
You first choose the line weight of your outfit based upon what type of fishing you’ll be doing.
If you’ll be fishing for panfish or trout, look at line weights from 4-6.
If you are going straight to bass, a 7 or 8 weight is better because it will deliver bigger flies and control bigger fish around cover.
An 8-1/2 to 9 foot length is average for most fishing. If you’re going to fish smaller streams with lots of overhanging trees, that’s when you might want to look at a shorter length.