This variable can be bound around a call to a process creation function. Note that setting the global value of this variable affects the size of all system processes too, so this is not recommended.
To create a process with a stack of 32000 words:
(let ((sys:*sg-default-size* 32000))
(mp:process-run-function "Larger stack" '()
#'(lambda ()
(print (hcl:current-stack-length)))))