How to know max specified memory in Oracle XE11g release 2?
The documentation is clear about this. See Oracle Database Licensing Information


  Oracle Database Express Edition  Oracle Database Express Edition
  (Oracle Database XE) is an entry-level edition of Oracle Database that
  is quick to download, simple to install and manage, and is free to
  develop, deploy, and distribute. Oracle Database XE makes it easy to
  upgrade to the other editions of Oracle without costly and complex
  migrations. Oracle Database XE can be installed on any size machine
  with any number of CPUs, stores up to 11 GB of user data, using up to
  1 GB of memory, and using only one CPU on the host machine. Support is
  provided by an online forum.


The 10g documentations shows explicitly: 1 installation, 1 instance on a machine. Compared to 10g in 11g you can use more userdata on disk.