If you previously specified a limit in a migration and want to just remove the limit, you can just do this:
change_column :users, :column, :string, :limit => 255
255 is the standard length for a string column, and rails will just wipe out the limit that you previously specified.
Updated:
While this works in a number of Rails versions, you would probably be better suited to use nil like in Giuseppe’s answer.
change_column :users, :column, :string, :limit => nil
That means the only thing you were doing wrong was using null instead of nil.