kiji-modeling: MODELING-7. Add size to RecommendationPipe

issue: https://jira.kiji.org/browse/MODELING-7
review: https://review.kiji.org/r/1558
commit: https://github.com/kijiproject/kiji-modeling/commit/cd0ad3030ac8d1d0ff8e9a4a4c6bd7cf08209aca