Random Riddle: Confusion Abounded

I’m not average, but was based on a mean.
My size, in theory, is constant: fifteen.

I’m two dozen steps, again in theory.
But walk my length and you’d get weary.

I take half and quarter steps at times.
In reality, I don’t follow the lines.

I shrink to nothing in two cold extremes.
Over a thousand miles wide in the betweens.

What am I?