How can I generate a WebM video in 720p?

I've called /api/video.webm to generate a short piece, and got an error in the status: 'Please subscribe to higher plan to generate higher resolution videos'. (Video ID: d4696ede6511484ba0d32d3ee3fd2c0c)

My subscription only supports 720p video. It is said in the doc that I can customize the dimension of the video, while there seems not to be a 'dimension' field in webm api. How can specify the dimension in webm API?