The map range node has nothing to do with the object size, it is just a simple function between the input and output, looking like this:
the lower plot shows the behavior of the node when “clamp” is off.
the “smooth”, “smoother” and “stepped” options change the shape of the curve.
Why I used the node: the x coordinate of the object output of the texture coordinate node roughly ranges from -3 to 3 here, because, in this case, this is roughly the size of your object.
We want, however, a value between 0 and 1 to feed into the color ramp node. So what I did was connect the setup and then played around with the “from min” and “from max” values until the gradient fit the object.