Mediaplayersprite is used for simple applications.
Please use MediaContainer for more complex applications (See helloworld8, for example).
You can inspect the SMP code for DisplayObjectEvent.MEDIA_SIZE_CHANGE events - they are indeed dispatched.
I don't believe that this is related to MediaPlayerSprite vs. MediaContainer, as I've attached listeners to the MediaPlayer and am polling its mediaHeight and MediaWidth. Furthermore, the aspect ratio does not change with the SMP player currently available for download, nor do I see events dispatched other than the initial resize event:
org.osmf.events::DisplayObjectEvent (oldHeight:NaN newHeight:352 newWidth:625 type:mediaSizeChange oldWidth:NaN )
The issue is that the aspect ratio of the content changes while playing back and the initial dimensions are no longer correct. For example: the initial aspect ratio of the content is 16:9. The player correctly starts showing the content at 16:9. The content switches aspect ratio to 4:3, but the player continues to display at 16:9 and there is no resize event triggered.
The size changes appear for MBR assets (please check the SMP debug.html page).
If you encountered it on adifferent use case, please raise an issue at http://bugs.adobe.com/jira/browse/FM with a code snippet allowing to reproduce it.