In order to find out the amount of memory, contact your site support group. There is an absolute limit on how much memory a given machine can have, but when the LPAR is set up in the system, an amount of memory is assigned to that LPAR -- which may be significantly less than the theoretical limit. Since your site support group set up the LPAR, only they should be telling you how much memory is assigned. There is a console command to display the high memory address, but that does not provide the usable memory for the LPAR -- and many applications programmers do not have authority to issue such console commands, anyway.
Perhaps more importantly, why do you want to know this? If you're planning on writing some program to use all of memory, you REALLY need to work with your site support group since such a program could cause some very bad system performance issues.