If you want to fully justify a single word on a line, or force Word to
adjust the spacing of letters as well as the width of spaces, you can also
"distribute" the paragraph.
Put the cursor in the paragraph and press Ctrl+Shift+j.
You'll have to adjust the font size yourself to make it look "nice".
Any other paragraph alignment (Left, Center, ...) will get rid of the
"Distribute" alignment. Or hit Chtl+Q (ResetPara) to remove it.
Greetings,
Klaus