To change the page file size in Windows Server Core this will need to be done using the command line.

To Show the Current Pagefile Usage

Run the following command:

wmic.exe pagefile list /format:list

To configure the initial and maximum size of the Pagefile

wmic pagefileset where name="c:\\pagefile.sys" set InitialSize=1000,MaximumSize=5000

Where the initial size and Maximum size is in Megabytes.  The above command is all on one line.