"Fit Content" button in property palette

[Ian Summerfield]

On the property inspector palette their are buttons to make a field 
"fit content".  This is exactly what I want to do when a user enters 
some text into a field.  I can't find out how to do it though!  How do 
I "borrow" the script behind those buttons on the property inspector?  
Or is it a property of a field to change?

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