I realize that this is an old issue, but give this a try:
Edit the door family. In the parameters set the width and height equation of the door size to be equal to the value (i.e. Width = 3', Height = 7', or whatever it may be). This will grey out the input value and not allow that value for the family type to be changed.
This does not mean they can never change that value. If they really wanted to they could go edit the family type and change it there. But it will keep them from changing it while view the height and width on the schedule.
I hope this helps you.
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