In general, longer skis are faster but harder to turn. In general, you add ski length to compensate for extra weight and extra height. You reduce ski length to compensate for lower ability. Still today's skis can be awfully versatile if you're skiing groomed runs. I'm 5'10", 230 right now and can ski just fine on 120cm skis, but my regular skis are 168cm.
50 lbs is a lot to put on in one year. And it's a lot of added stress to put on your knees relatively suddenly. You could have started wear and tear on your knee last week and then aggravated it this week. Or it could be the extra length ski caused you to start forcing your turns and that was what made the difference. What matters most is that your knee is telling you that you need some help. Find a doctor, physical therapist or trainer who can test your strength and recommend some exercises before you get hurt. Muscle strength imbalance is a significant contributing factor to ACL/MCL tears. Having an instructor look at your turns (or posting video for us to look at) could also help identify technique changes that can reduce knee stress. Obviously, losing some weight would help, but some of us know that's easier said than done. You may want to get a neoprene brace for short term protection, but please don't use one as a long term solution.
Please take care!