the parameteres which most affect the SGA size are :
db_block_size : size of an oracle (or data) block, in bytes
db_block_buffers : number of database buffers, each the size of db_block_size
log_buffer : number of bytes in the redo log buffer
shared_pool_size : self explicit, in bytes

if you do sho sga under SQL+, you'll see the SGA size and what it's made of. you can also look in v$parameter for above parameters and compute your SGA size