it may be wrong, but i think, when number of inner pages of any sites is too less(approx 4 to 5), then we can say that that is small sites.
Micro sites Mircrosites are mini websites dedicated to delivering information and soliciting 'call-to-action' responses for particular products, services, market segments, demographic groups, marketing campaigns, initiatives or promotions, Microsite traffic can be generated from e-newsletters, email campaigns, links from corporate websites, online and offline advertising. direct-mail campaigns, and Google-type advertising vehicles.