The size limits of database objects (such as tablespaces, etc) are contained in the SQL Limits Appendix of the SQL Reference Vol 1.
V7 is out of support by more than 2 years.
I don't know what you mean by "I/O buffering even though when application just started but no users conencted to the application." Memory for DB2 bufferpools is allocated when the first application connects to the DB2 database. You can see whether there is a connection with the following command:
db2 list applications show detail
Note that this shows all connection to the instance, but the db name is included in the list.
If your bufferpools are large, I would increase the dbheap significantly. In V7, the size of the dbheap must be increased as the bufferpool size increases, unlike V8 or V9.
M. A. Feldman
IBM Certified DBA on DB2 for Linux, UNIX, and Windows
IBM Certified DBA on DB2 for z/OS and OS/390