This is an answer that won't be an answer.

It depends on many factors. There is no single "out of the box" answer. You'll start with something and tune it from there.

o Going to run lots of PLSQL? (eg: replication, Oracle apps, your own stuff with tons of PLSQL) - need a large shared pool

o Going to run Multi-Threaded Server? Need a large shared pool

o Going to run Java, lots of it, in the database? Need a large java shared pool

o Going to have 10 users doing lots of really big queries? Need larger settings for sort area size, hash area size and such -- these might not affect the SGA (but they might if you use MTS)

o Going to have 1000 users doing OLTP? The mix will be different.

o Going to have anything else on this box? They will need memory.

The settings that *most* affect the SGA size will be:


o db_block_buffers -- how many buffers to cache in the SGA.
o shared_pool_size -- if you do not have a large pool allocated
this will be used for that as well.
shared sql, loaded plsql and so on are in
here.
o java_pool_size -- when you use java, this is used.
o large_pool_size -- used by MTS, backup, and so on for large
chunks of memory that should be freed on
demand, not managed LRU like shared pool.

Thing that will affect it to a lesser degree:

o log_buffer -- redo log buffer, measured in Kbytes.