import clr
#Import the Revit API
clr.AddReference('RevitAPI')
import Autodesk
from Autodesk.Revit.DB import *
#Start scripting here:
walls = UnwrapElement(IN[0])
output = []
for i in walls:
output.append(UnitUtils.ConvertFromInternalUnits(i.Width, DisplayUnitType.DUT_MILLIMETERS))
OUT = output
I was convinced @Andreas_Dieckmann had this in his portfolio 
I can see theres one in Steam but it returns the width in meters:
