I just pulled 20x out of the air. I would make it longer than 10x - 11x would work. It can be as long as you want it as long as it is longer that 10x. I would put the thermocouple at least 10 diameters away. The reason behind this, other than fully developed flow, is that you have different temperatures at different locations coming out the back of the radiator, so you want to get a good average. You can test that to by taking readings at different positions. If you want to get really specific, I would make the duct, get a small diameter (very small) metal rod or wood dowel, put a hole in the duct for the rod, attach my TC to that and position it in the middle of the duct. You can get a compression fitting at the rod's diameters that would let you lock it down in the desired position.