sizing of PGA_AGGREGATE_TARGET parameter.

Hi,

My SAP is on oracle 10g. It ic currently having a PGA_AGGREGATE_TARGET parameter sized at 25MB. The earlywatch report is giving a warning for the current size. . In EWA report, the ideal value is given as 20% of total memory available for oracle. could you please suggest how to determine the right size for this parameter or even how to find out the total memory available for oracle.