I'm quite certain that a high PersonRank on Google Blogoscope is directly proportional to the ammount of posts you make. It seems unlikely that anything else, such as post length is calculated. Possibly Philipp would like to comment on this??

The PersonRank is a household secret – I do not want to invite SEO spammers trying to make a quick buck by artificially boosting their PersonRank.

Seriously though, yes, it's directly connected to a person's number of posts, but the dynamics may change if everyone has a PR10 :)

Just letting you know ;).. so you can be prepaired to start cranking out another algorythm.