The [embed] shortcode is a WordPress shortcode so I don’t actually know much more than what listed in the FAQ and on the page that is linked to in the FAQ.
I see that is says this on the WordPress [embed] doc page:
“It also allows you to set a maximum (but not fixed) width and height, like so: [embed width="123" height="456"]…[/embed]”
So your syntax looks correct.
Maybe the issue is the use of the short URL instead of the long one?
Or maybe you’re video is not available in a smaller size?
I also found a few issues like this with the [embed] shortcodes if the Jetpack Plug was installed. So, if you have that installed try deactivating it.
Hope that helps.