The outer sheath should be long enough to reach the carbs without creating any kinks or sharp transitions do to stretching. Kinks bind. Smooth transitions operate freely.
A little extra length isn't a problem. Truly excessive length will still work, but it can be a problem from a housekeeping stand point. If the outer sheath is metal, make certain any excess length can't flop around and contact anything electrical... like the starter solenoid. Of course, keep it away from the belts and pulleys. Nylon cable ties are a good thing.
If the inner steel cable is too long, cut off the wretched excess. The end should protrude through the linkage clamp by a comfortable margin, but an excessively long pigtail can catch on things and bind. Keep the pigtail down to a couple of inches or less.
Cutting the inner cable can be difficult. Using side cutters causes the strands to separate and become a mess that's a pain to get through the linkage cable clamp. Side cutters are a desparation tool. A small cut-off wheel in a Dremel tool is the best.
Regardless of the tool of choice, I've found it useful to solder-tin the end first. Pre-fit the cable to the linkage. Mark where the cable clamp will fit and where you want to cut the pigtail. Then remove it.
Grasp the inner cable at it's end and a point a few inches inboard of where the cable clamp will pinch it, then stretch it taught/ straight. Apply flux to a span from about an inch further inboard of the clamp point out to a comfortable distance beyond the cut-off point. Tin it with solder using plenty of heat. A little pen-style, PC board soldering pencil isn't enough. A gun-style soldering iron is called for... or a small propane torch.
Hold the cable taught until the solder solidifies.
Solder at the cut point will minimize fraying of the cable end when cutting and in the future. Solder at the clamp point will minimixe crushing and distortion. The soldered cable will be much stiffer than before, but it can still be bent easily by hand if need be.
I've also had good success using radio control model fittings. Small cable controls are common for RC airplanes in a variety of styles. Some use a steel cable or music wire sliding in a plastic tube. Then there are a variety of fittings that can be soldered onto the end.
The fitting I like is brass solder fitting about 2 inches long (made by DuBro). One end, the short end, is female and a snug slip fit over the cable. Solder it on. The rest of the length necks down to about the same diameter as the cable and is either a threaded brass rod, or solid for soldering on other fittings. I prefer the solid rod since it's stronger (threads cut into it). Slip the brass rod through the carb's linkage clamp and tighten down. I've had one on the throttle cable in my Lotus Eclat for over 10 years... trouble free and no frayed ends. However, it's reliability will depend upon your ability to solder. Make an honest judgement call.