Most modern skis are capable of being skied at way-too-fast speeds, assuming you have the skills to properly ski at those speeds in the first place. That is, the speed at which most modern skis start becoming "unstable" is a lot faster then most people will ever want to be skiing.
That said. Generally speaking, the longer the ski, the more stable it becomes. Real downhill racing skis -- i.e., skis built for eye-watering don't-fall speeds -- are long.
Speed stability also depends on what type of terrain you're skiing -- skis built for hauling in deep snow and skis built for hauling on groomers are two different animals.