According to http://mathworld.wolfram.com/RadiusVector.html the radius vector (often written as r hat, or the letter r with a carrot ^ over it) is just the distance from the origin to the point of interest.
So the magnitude is the distance between the point and the origin, and the direction is the direction from the origin to the point.
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Double the Radius to Calculate the Diameter.
how to calculate the elbow radius or elbow length
From what? You need other variables you can use in the standard circle formulae from which to calculate the radius.
The "triple product" is defined for vectors. You can't calculate a triple product if you don't know the components of the vectors (or some other information, that allows you to calculate those).
diameter = 2 * radius