SGA can actually be as big as you want as long as you avoid paging and swapping.
saran - depends on database requirements as to how you size SGA. To start with, you need to run some scripts on dictionary cache, library cache, buffer pool, redo log buffer and check hit ratio's to decide how to size these areas.
I think you will find these scripts at this site. They are also posted all over the web.
Once you have eliminated all of the impossible,
whatever remains however improbable,
must be true.