Holy cow, I wrote a book!
The keyboard shortcut for resizing all columns in a report-mode
(also known as Details mode) list view control
to fit the current content width is
That's the + key on the numeric keypad.
(If you're using Explorer, you can also right-click the column header
and choose Size All Columns to Fit.)
Note that this command is
a verb, not a state,
so it takes into account the contents of the listview
at the time you press the hotkey.
If the contents change and you want the columns resize
based on the new contents,
you'll have to press the hotkey again.