CONFIG_LOWMEM_SIZE_BOOL is not available for thedefaultarchitecture x86.
Result is shown for architecture powerpc

Set maximum low memory


Linux Kernel Configuration
└─>Advanced setup
└─>Set maximum low memory
In linux kernel since version 2.6.15 (release Date: 2006-01-02)  
This option allows you to set the maximum amount of memory which
will be used as "low memory", that is, memory which the kernel can
access directly, without having to set up a kernel virtual mapping.
This can be useful in optimizing the layout of kernel virtual

Say N here unless you know what you are doing.