I was just about to post this, but you beat me to it.
The page you linked to isn't loading for me. Here is another link though.
“A key feature is that you need a relatively large treadmill to simulate natural walking,” explains Ernst. “The one that will be demonstrated is 6m by 6m, with an active walking area of 4.5m by 4.5m.”
According to Ernst, this is the minimum size necessary for 'natural walking'.
Since Star Trek holodecks use a treadmill effect to simulate walking, I guess we now know what the minimum size of a holodeck is.