The length of the skis has more to do with height, weight, and the type of skiing you like to do. Its well known that shorter skis make quicker turns and are more agile while longer skis are more stable at high speeds. Im no ski mechanic but when it comes to knee injuries, Id focus more on how you have your bindings set.
"One ski boot. One Glove. Nipple shirt. Thats some futuristic shit right there." -S.Mouse!