I dont have any elbow guards, but just recently tried out two different knee guards. I'm 6' even. 255lbs.
17.750" (45.09cm) 4" below knee 21.5" (54.61cm) 4" above knee
Guards are a little hard to come by in that size. I've tried two types of knee guards.
Leatt Airflex Pro (XXL) This fit well and provides a basic level of protection. The pad on the knee is pretty thin, but should protect against most things other than landing on a sharp rock or something. It also has some nice pads on the top and sides of the knee. Unfortunately, one of the guards started to rip in two places after only 4 rides. So I sent it in for warranty consideration.
POC Joint VPD 2.0 (XL) This one fit well, although a little tight. It felt much more substantial in the protection dept. The pad is like a thick hard memory foam. It covers the the knee and a good portion of the shin. It also has an extra pad on top and the main pad is fairly wide, which adds a little bit of side protection. The downsides are that it's a bit warmer than the Airflex and the material in the back tends to bunch up. I've only had one ride in these. The negatives are definitely noticeable, but I didnt really mind it after getting warmed up. I much prefer having them on. I really like how substantial the protection feels. Much better than the Leatt, while not going to the hardshell cap extreme.