As a general guideline, on a Sun SPARC station, estimate that:
A newly installed Replication Server uses about 7MB of memory for data and stacks.
Each DSI connection adds about 500K. This value increases if you configure larger values for MD memory (md_sqm_write_request_limit configuration parameter).
Every RepAgent connection adds 500K. This value increases if you configure larger values for SQT memory (sqt_max_cache_size configuration parameter).
If you have thousands of subscriptions, or if you increase the function string cache size, you must account for that by adding more memory.
Memory for subscription rules is a function of the columns referenced in the subscription and the number of rules. A typical subscription adds less than 80 bytes plus the combined size of all subscription predicate values.
Function-string cache size can grow to 200K.
Remaining system table cache size is based on the number of objects defined. Each replication definition consumes memory equal to approximately 250 times the number of its columns. This might be a factor if you have numerous replication definitions with many columns.
