How do I find the length of an array within a shader before setting it from a script?

I've been coding a grid-based warping shader to correct projected images. The shader exists as an image effect. Everything is working great, but there's just one problem. Here's the declaration of the arrays that I use for the data within the shader...

 static const int GRID_SIZE = 15;

 float _WarpUDisplacements[GRID_SIZE * GRID_SIZE];
 float _WarpVDisplacements[GRID_SIZE * GRID_SIZE];

In order to reduce the likelihood of bugs within my code, I want the arrays I pass into my shader to have the same length as the arrays within the shader. Being as the grid size is defined within the shader, I figured I might be able to do the following from my C# script...

 int warpArrayLength = this.warpMaterial.GetFloatArray("_WarpUDisplacements").Length;

However, I'm getting an exception, as the returned array is null for some reason. Is there any way that I can get at either the lengths of the arrays within the shader before I set them, or to access the GRID_SIZE constant declared within the shader?