linum-format

linum-format is a variable defined in `linum.el'.
Its value is

dynamic

Documentation:
Format used to display line numbers.
Either a format string like "%7d", `dynamic' to adapt the width
as needed, or a function that is called with a line number as its
argument and should evaluate to a string to be shown on that line.
See also `linum-before-numbering-hook'.

You can customize this variable.