From the decker reference manual,
If the grid has headers and more than one column is displayed, you can drag the space between column headings to resize the columns. By default, every column is given equal horizontal space. You can restore default spacing by clicking "Reset Widths" in the grid properties dialog.
The .widths property is described in the Grid Interface section, along with all the other scriptable properties:
x.widths: A list of up to 255 widths of table columns, in pixels. r/w.