Classifying text documents using linear/incremental topics
If you want these output dimensions to be continuous, simply convert your size and relevance metrics to real-valued targets.  Then you can perform regression instead of classification, using any of a variety of models.  You could even attempt to train a multi target neural net to predict all of these outputs at once.

Additionally, you might consider first using  a topic model such as LDA as your feature space.

Based on the values, it sounds like the "relevance" might be a variable best captured by techniques from sentiment analysis.