How can I change the default width of Finder columns?
In column view in the Finder, you can resize columns by positioning the mouse pointer over over the thin line that separates column panes, and dragging it to the left to make it narrower or to the right to make it wider.

To make the Finder remember the new width, hold down the option key(⌥) while dragging the pane separator to the desired width.

The best documentation I can find on this from Apple alludes to the fact that holding down option resizes "all columns at once" but does not explicitly say that this makes the change persistent.