It will always render the full media file, regardless of it's size.
Doing the same with the thumbnail-tag provided by SonataMedia lets you define different sizes which than will be rendered in size you would have configured. Take a look at introduction to SonataMedia's context configuration.
I dunno which answer you are looking for, since this question can be answered in different ways, so I'm trying to answer all of those ways.
Getters on Media objects
If you're just interested in getting the values from your media object, you can just simply call:
Media rendering
Sonata's twig-extension for rendering medias will automatically add width/height to your html.
Media Contexts
It will always render the full media file, regardless of it's size. Doing the same with the thumbnail-tag provided by SonataMedia lets you define different sizes which than will be rendered in size you would have configured. Take a look at introduction to SonataMedia's context configuration.