It was discussed in #1

The best option is to determine the size of contents and clip the board automatically
As a quick fix, you can just let the user control the width/height values via the GET parameters
Example: http://img.catbo.at/oembed.json?url=http://img.catbo.at/3fb93c1b7891aedb18aa41fb325cbc92e82e58a1.jpg&maxwidth=200&maxheight=45&format=json