I was poking about in the RSS 2.0 Spec and noticed this restriction when talking about the <image> element:
Maximum value for width is 144, default value is 88.
Maximum value for height is 400, default value is 31.
Where do these numbers come from? It seems like an arbitrary decision. I should be able to include an image as big as I like, it's up to the consumer to decide whether or not to display it.
