To change the indentation for a numbered list, click the first list symbol; you'll notice that Word selects all items of the same list (level). Right-click and choose Adjust List Indents from the context menu. For a single-level list, you'll see the Adjust List Indents dialog box; for a multilevel list, you'll see the Multilevel List dialog box.
For a single-level list, you change the "Number position," "Text indent," and/or "Add tab stop at" as desired. "Number position" determines the position for the list number. "Text indent" determines the starting point of the second and subsequent text lines. "Add tab stop" determines the starting point for the text following the list number. Note that all values refer to the left margin.
See also the "Equalizing the space" section of the article at
http://wordfaqs.mvps.org/NumberAlignment2007.htm.