Resolution:
The default settings for java properties can be viewed and modified in the Administrator GUI:
In the Service Instance configuration for the application, select the Server Settings tab, and set the value as desired.

If the need to permanently modify these values arises, this can be achieved by specifying defaults set in the EAR file at design time.
Add to &ltTibco_Home>/bw/&ltversion>/lib/com/tibco/deployment/bwengine.xml the following properties:
                        &ltproperty>
                           &ltname&gtMinimum Heap Size</name>
                           &ltoption&gtjava.heap.size.min</option>
                           &ltdefault&gt512M</default>
                           &ltdescription&gtSets the minimum heap size</description>
                        </property>    
                       &ltproperty>
                           &ltname&gtMaximum Heap Size</name>
                           &ltoption&gtjava.heap.size.max</option>
                           &ltdefault&gt1024M</default>
                           &ltdescription&gtSets the maximum heap size</description>
                        </property>
After (re)building  the EAR file and uploading it in the Admin GUI, those modified Engine properties should be seen under the category "Adapter SDK properties" in the Advanced tabe of the Service level (.PAR) configuration.

How to permanently modify the default heap size settings for BW service instances?