Show PGA sort area and hash area sizes for each session.
select &hwm*(2048576+p1.value+p2.value) pga_size, &hwm*(2048576+p1.value+p2.value)/(1024*1024) pga_mb from v$parameter p1, v$parameter p2 where p1.name = 'sort_area_size' and p2.name = 'hash_area_size' ;
The PGA_MB column gives the number of megabytes of memory Oracle will use for each connected database session. Multiply this by your expected peak number of sessions to figure out how much RAM to set aside for your sessions; alternatively, look at how much RAM you can spare for sessions, divide by PGA_MB, and set the initialization parameter SESSIONS to not exceed that value.