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

Set custom user task size


Linux Kernel Configuration
└─>Advanced setup
└─>Set custom user task size
In linux kernel since version 2.6.15 (release Date: 2006-01-02)  
This option allows you to set the amount of virtual address space
allocated to user tasks. This can be useful in optimizing the
virtual memory layout of the system.

Say N here unless you know what you are doing.