Hi,
The size of Sga normally depends on various factors:
1)amount of RAM available.Normally the SGA size is kept to 60-70% of available RAM in your system.
2)Also the SGA is tuned by taking the cachehit ratio percentage,Library Hit ratio,RowHit ratio.If any of these ratios fall or is in access of the percentage given by Oracle,then you increase or decrease the SGA size.
3)Also Yes it depends on the transaction activity of the business.
4)No of users accessing the database.
But the major optimal Size of the SGA in the database is tuned according to 1 and 2.

Regards,
Rohit Nirkhe,Oracle/Apps DBA,OCP 8i
[email protected]