What object property or drawing property would the Field Expression reference? Field Expressions cannot reference arbitrary points in space (and in any case, these would be constant hence the Field would never update), and it appears from your code that you wish to create a Field to display two lengths calculated from arbitrary points in space.
I'm guessing that you in fact wish to select an object in the drawing and create a Field referencing the rectangular length and width of the object, however, since the such dimensions are not found in the properties of any drawing objects, there is no object property for the Field Expression to reference.
If I have indeed understood your intentions, I believe you would need to use an Object Reactor to monitor modification to a notifier object and update the MText accordingly in the reactor callback function.
PS:
Formatting code in your posts.
