Well, you could use the format statement... Something like this in the AfterUpdate event of the field...
Me!YourField = Format(Me!YourField, "!&&&&&&&&@")
The bang(!) causes left to right place filling, the ampersand(&) causes a character to be displayed or nothing, and the At(@) will serve as a placeholder, either a character or a space... This is sorta sloppy, but should work... You should add as many &'s for as long as you think the field would be... If you need more spaces, add more @'s... Hope this could help you out..
Doug