The dialog for setting properties of Impress styles does not include a pane with the settings accessible through Format | Character on the menus, in the Position pane. Example: Percentage of default glyph width to use.
This setting can be made by selecting an object with the relevant style, making a manual change through Format | Character, then pressing the Update Style in the Styles sidebar.
The same issue was filed in bug 82664, which itself is a duplicate of bug 40871.
The Format > Styles > Edit Style command (or using the context menu in the Stylist at the sidebar) deals with the paragraph style, ie. Title, Subtitle etc. We do not yet have access to character style in Impress.
*** This bug has been marked as a duplicate of bug 82664 ***
Let's focus on the solution, bug 40871
*** This bug has been marked as a duplicate of bug 40871 ***
*** This bug has been marked as a duplicate of bug 128810 ***