change Alphamap and Colormap option of volshow to have better representation for your application. there is a simple example in documentation of dicomreadvolumein MATLAB documentation.
about the height. it will show your stacked data. for example here i stack 100 image together.so for that you should have information about sample distance between each slice of your real example.
for width and length it is possible to measure them in pixel from image. you can use imdistline or select corner pixels (or with corner detection automatically detect them) and compute distance of them. but all this results would be in pixel unit. you should have minimum information about sensors and experiment setup to translate this distances to metric units.