You can use -Xloratio parameter For WebSphere Application Server V6.0, V5.1 and V5.0 (Java 1.4.2) .If your WAS version is WebSphere Application Server V7.0, V6.1 (Java V1.6, V1.5) ,There is no need to explicitly set the size. The value can be manually limited with using the -Xloainitial and -Xloamaximum, however it is not recommended.
The detailed technote :
http://www-01.ibm.com/support/docview.wss?uid=swg21236509&myns=swgws&mynp=OCSSNVBF&mynp=OCSSEQTP&mync=E