Unfortunately, your question is reminiscent of the one that starts: 'how long is a piece of string'? Everyone has their own ideas about what suits them best and of course the shape, and size of the bed is derived from the manufacturer's idea of the type of van he is producing. Smaller vans have smaller beds usually to make the available space look larger.
What I would suggest is look at the bed you use at home and go for the size that is closest to that. The cut-off, as long as it is not too severe, is not usually an issue.