I couldn't find a way to resize fields build-in per drag & drop.
Most commonly (Excel, ...) this is done by two ways:
1. Resizing the width per drag & drop by clicking on the bar between two fields (splitter).
2. Double click on this bar (splitter) resizes the field to the minimum size of it's content.
I often see truncated fieldnames or values or much to wide fields in the Kendo UI PivotGrid. Eventually this is something important to improve.