Here are the post rules from the upgrade section :

Post Count Upgrade

10 POSTS = 50 MB Web Space/1000 MB Monthly Data Transfer
50 POSTS = 50 MB Web Space/5000 MB Monthly Data Transfer
100 POSTS = 60 MB Web Space/5500 MB Monthly Data Transfer
400 POSTS = 80 MB Web Space/6500 MB Monthly Data Transfer
800 POSTS = 100 MB Web Space/8000 MB Monthly Data Transfer