I think it's in the bottom of your gcode file. Open it in a text editor and scroll to the bottom. It might be in cm^3 since that's the typical unit, but just multiply that value by 1000 to get mm^3.
Another trick you can use is that the plastic weight that is displayed in the preview window is just the [volume in cm^3] * [density in grams/cm^3]. So if you go to the Other tab and enter a density of 1000, then you will get [volume in cm^3] * [1000 mm^3 / 1 cm^3] = [volume in mm^3]
So using a density of 1000 will make the "Plastic weight" in the preview window actually equal to the volume in mm^3
