For such customization I recommend you modifying the template of PropertyGridFieldTemplate instead. Within it there is a Grid with x:Name="PART_PropertyGridFieldGridN" with three column definitions. The first one is for the PropertyGridPresenter, the second for the Label and the third one for the editor. Basically, you can achieve your goal by setting MinWidth/MaxWidth properties for the needed element.
I have attached a sample application as a demonstration.
Regards,
Stefan X1
Telerik
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