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Apologies for the late reply. I'm guessing you want to have access to the Width and Height of the textbox from within your model classes where you can use them in calculations? I'm wondering if the best way to do this would be to create an attached property. It could be called something like SizeProvider and binds to a model object that can store the size. Within the attached property, you can listen to the size change events of the textbox, and update the size value stored in the model object.
Let me know if you have further questions about this.