Replying to my own question, since v27 of the Word template, it looks like the auto-rescaling is disabled by default, but I don't know exactly why.
To re-enable it, you may use the following syntax:
but it may not always work. In that case, you have to fall back to manual rescaling, like in the following example to 70%:
Rem: another manual rescale is also available in any environnemnt, see http://plantuml.sourceforge.net/commons.html#scale