The ski is too short when the soft snow platform under it will not support the load from your turn and you slide sideways instead of base-carve arc-2-arc when you would be carving arc-2-arc on a longer ski.
The ski is too short when it fails to smooth out the bumps and demands your full attention to stay on top of it with regards to fore-aft balance when skiing choppy snow or making GS turns in small (less than knee high) bumps, or encountering unexpected bumps (e.g. snow gun whales with your goggles full of snow gun spray).
The ski is too short when it fails to provide the float YOU WANT, and a longer model in that ski will provide that float.
IMHO, a ski is too soft, not too short, when it folds up on you.
In the old days, circa 1983, longer skis were needed for stability, with a noticeable difference between lengths differing by 5 cm. Now not so much; it's more a function of the ski design. I'm not sure when that changed, but it did. Now a longer length in an unstable at speed (say 55 mph) ski in the middle length is even more unstable and harder to manage in its longer length, at least on hard snow.