If you're a careful reader of the YouTube API Terms of Service as well as the YouTube Player documentation, you may have noticed that while embedded players smaller than the minimum size might not support all player features, we haven’t defined what that size is.
As a part of our spring cleaning, we've tidied up our documentation to specify a minimum player size, which is 200px by 200px.
This change will take effect beginning late April 2012, so please check your application to avoid surprises. If you have any questions or comments about this, or any other YouTube API feature, please let us know on the API forum.
-Jarek Wilkiewicz, YouTube API Team