Update: the above only obtains the heap size if a custom value has been explicitly configured. From this other post, here's a mechanism which will obtain the maximum heap size otherwise (note there doesn't appear to be an attribute to obtain the minimum heap size):
I sometimes need to set a custom JVM Property for one or more Application Servers and dislike the tedium of setting them one-at-a-time through the console.
Here's a Jyth
wsadmin -f addJvmProperty.