Hello,
Which version exactly are you running? Java heap size limit is shown but no longer relevant on 7.3 E0703 and later versions. Prior to that, you should adjust it to about 1/4 to 1/3 of the total memory available.
How much you need to assign depends on the needs of your installation, based on the requirements outlined in the sizing guide linked here.
Note that the system requirements are cumulative for optional modules - so if your requirements are 16GB memory for the base platform and 8GB for NTA, and you are running both on the same system, you would need at least 24GB of memory in total, along with 6-8GB Java heap size configured (if relevant).
As for determining the heap size usage - you can compare the amount of memory the 'jserver' process is using to the limit configured. If it is at or barely below the limit, you likely need to assign more heap memory. Another way to view heap memory usage is to run iMC\client\bin\jmonitor.bat script, which will open a Java Monitoring and Management Console that allows you to see the exact heap usage.