Haha, was just messing around with this, and the ForgeTypeId. Got way more complex then the actual answer 
That being said though, I think the OP needs the ConvertFrom
import clr
clr.AddReference('RevitAPI')
from Autodesk.Revit.DB import UnitUtils, UnitTypeId
wallType = UnwrapElement(IN[0])
widthInternal = wallType.Width
#output unit
uid = UnitTypeId.Millimeters
OUT = UnitUtils.ConvertFromInternalUnits(widthInternal, uid)
Image for context
