If I've got my head around it right you should be able to print a 1280x1024 web snap at about 100x80mm at 300dpi.
You might get away with 150dpi in which case it would be a bit larger. Depends how it's being printed really, if you know a local digital printer you could maybe run off a few experiments at different sizes and resolutions.
What about taking a photograph of the screen with a digital camera - as a kind of 'arty' presentation method?
