CONFIG_DEFAULT_MEM_SIZE is not available for thedefaultarchitecture x86.
Result is shown for architecture xtensa
default or selected kernelversion does not have config value CONFIG_DEFAULT_MEM_SIZE.
Result is shown for kernelversion 6.7-rc3

Maximal size of the default memory area


Linux Kernel Configuration
└─>Platform options
└─>Maximal size of the default memory area
In linux kernel since version 3.19 (release Date: 2015-02-08)  
This is the size of the default memory area.
Default memory area has platform-specific meaning, it may be used
for e.g. early cache initialization.

If unsure, leave the default value here.