Uploading a copy of the 'Shuttle flip.webm' from the prototype page to my local test wiki with transcoding working, I end up with two playable file versions:
"WebM source" at the original 1080x608
"WebM 480P" -- which is actually sized at 640x360
Since HD video formats are defined by their height, not their width, it might be better to go ahead and size "480P" to 480 pixels high and "720P" to 720 pixels high etc, rather than "480P" to 640 pixels *wide* and 720P to 1280 pixels wide.
A 480P transcode of a 16:9 file should probably either be 854x480 (square pixels), or else use 720x480 and tweak the aspect ratio.
Note that a 360 pixels-high file would not be terribly in plenty of circumstances, but should be labeled as 360P.