You will find everything you need in Connections Wiki (
http://www-10.lotus.com/ldd/lcwiki.nsf/dx/Administering_Lotus_Connections_lc3). To summarize :
- Files (community files also) and Wikis : media max size in the related xxx-config.xml file, collection max size in DB : you have to modify everything with wsadmin.
- Blogs : you can set the parameters in the administration tab
- Activities : in oa-config.xml (use wsadmin)
- Forums : in forum-config.xml (use wsadmin)