pixelW and related questions

[lrocax]

Based on this question (https://stackoverflow.com/questions/59308919/radiomics-package-in-matlab)

I used is this code:

volume = double(rgb2gray(imread('http://www.cs.sjsu.edu/~bruce/images/fall_2016_cs160/lectures/eye_pupil_tracking/grayscale_eye_cropped_to_bounding_box.png')));
mask = ones(size(volume,1),size(volume,2));
[ROIonly,levels] = prepareVolume(volume,mask,'Other',10, 22, 1, 22,'Matrix','Uniform',256);
[GLCM] = getGLCM(ROIonly,levels);
DataGLCM = getGLCMtextures(GLCM)

 
I do not know what is the pixelW so what will be the value? In the case of this png file what shell I use as "Numerical value specifying the in-plane resolution (mm) of 'volume'"? (I used a random number of 10), What shell I used in scale as it is related to pixelW?. When I will use it on eye images I will use the full image like in the case above where mask and volume are the same.
 
Thanks a lot,
Lluis