The width of a space is determined by the font name and font size; there is nothing you can do to change it.
Using a tab stop and indentation, you should be able to specify the appropriate distance(s) for your list format. Take a look at
http://wordfaqs.mvps.org/NumberAlignment2007.htm. For more on setting up numbering in Word, see
http://www.shaunakelly.com/word/numb...g20072010.html.