file: /proc/sys/kernel/sgbigbuff
variable: kernel.sgbigbuff
Official reference

This file shows the size of the generic SCSI (sg) buffer. You can’t tune it just yet, but you could change it on compile time by editing include/scsi/sg.h and changing the value of SG_BIG_BUFF.

There shouldn’t be any reason to change this value. If you can come up with one, you probably know what you are doing anyway :)