I've been having knee problems lately too. Easter weekend I damaged the medial collateral ligament on the inside of my left knee. Don't know how I did it, so it must have been one of those "boy, that hurt" times, but then I guess I just went on about my business By that evening though, it was really hurting and has been painful ever since.
My doctor first gave me a straight-leg brace which helped, but I had a really hard time walking, getting in the car, etc. So he then recommended one from Wal-Mart of all places. This one was much smaller, easier to use, and allowed my knee to bend. Best of all, it only cost about $20. It's made of a stretchy fabric and has two sections which fasten with velcro behind the leg. On each side there is a metal hinge that allows the knee to bend, but doesn't allow movement from side to side so keeps the knee more stable.
I'm quite overweight too, and also have a big thigh. Both of these braces tended to slip down when walking, but the Wal-Mart one could be tightened easier to help with that. I have used it much more than the other one. And happily my knee is finally starting to feel somewhat better. I still have a lot of pain, but I can tell there is improvement. Boy am I glad too, because I'm really tired of being injured. Also, the doctor said that if it doesn't get better, the only option would be a referral for surgery, and I certainly don't want that.
I don't know if a brace like the one I got at Wal-Mart would help you like it did me because my problem wasn't in the joint, but it could possibly help. Unfortunately, you may end up requiring a custom-fitted brace.
I hope this helps.