How to get length of inner array in unityscript multidimesional array?

I have a multi-dimensional array (using built-in array's) in unityscript. I define it programatically like so:

 var sequenceArray : Vector3[ , , ] = new Vector3[width, width, width];

I add values to it like so:

 sequenceArray[x,y,z] = cubePosition;

Elsewhere in the code, I need to be a able to find the length of the inner arrays (ie the value of width), using something like the following:

 sequenceArray[0].Length

But I get the error:

  'Length' is not a member of 'UnityEngine.Vector3'. 

I come from a JavaScript mentality, where a multidimensional array is just an array of arrays, and I think that is what is confusing me here. Is it possible to even get the length of an "inner-array" in a multidimensional built-in array? Or is there no such concept of "inner-array" when using a built-in array in Unity?

Thanks for the help.