Monty,
I would normally do 15 - 20 sq m. Here's why.
By far the hardest thing is getting in front of the customer in the first place.
Once there, you've got a golden opportunity to let them see what you can do to THEIR carpets. In other words they can see what the result is, rather than just telling them.
If you look at the video that Mike recently posted or this old one of mine
http://www.youtube.com/watch?v=0ZX-vhqFj2o , you can see what can be achieved.
The evidence is there if they have to refer the buying decision up another couple of levels. I have hardly ever lost a job after a 20 sq m demo.
Basically, what I am saying is, don't waste that face to face opportunity.
Garry