1. Log into EM (Enterprise Manager for OAS) Usually the http://192.168.1.1 (root www directory on your JAS server)
2. Click on the instance that you want to know the heap size
3. Click on the Administration tab on the top
4. Click on Server Properties link in the first section of Administration tab.
5. You will see initial and maximum heap sizes.
You can enable tracing below and see what the heap size reaches at a maximum, or your Server Manager configuration should also have this information per instance.