How to get the memory occupied by the knowledge center documents in Appian server?

You could try using this: https://docs.appian.com/suite/help/24.4/fnc_scripting_knowledgecenter.html though I'm not sure the information is very reliable. A sure file method is to open a case with Appian Support where they can give you the breakdown for the disk space usage between the different components.

If you have an admin user, you can try using sizing script

https://docs.appian.com/suite/help/24.3/Using_the_Sizing_Script.html

If you know the names of all the KCs, you can use the script referenced in this thread.

community.appian.com/.../size-of-all-kcs-in-the-environment