One way to encourage longer articles would be to rank languages on the www.wikipedia.org front page by word count (or perhaps byte count) rather than article count. According to [1] the Chinese Wikipedia has 50.6 M words in March 2007 and the Russian has 47.1 M words, compared to the Swedish Wikipedia's 36.2 M words. Changing the ranking of the Swedish one from 9th to 11th would send a clear message to the stub-happy swedes.
The amount of information in each word varies from language to language, so that doesn't work too well. It works for comparing similar languages, but Chinese and Swedish could end up having very different numbers of words for exactly the same content.