To be fair, I don't think he is going to be able to tell much just by playing around with the laptops at the store. It's something you really need to try out for 1-2 weeks at least before you can conclude whether the size really fits you or not.
One irritating thing is that those display macbook units come only with the stock apps. It would nice if they could load Office (or at least install iWork's, now that it's free), so users can have a better feel of well the screen sizes lend themselves to the various productivity tasks. You can only tell so much by messing around with Safari and Notes.
As a general rule of thumb, I would recommend the 13" model if you really plan to do a lot of typing (because of the extra vertical screen space).