I am not sure if this is the answer to your question but I found something in the SEGGER embOS generic manual:
Before embOS takes over control (before the call to OS_Start()), a program does
use the so-called system stack. This is the same stack that a non-embOS program for
this CPU would use. After transferring control to the embOS scheduler by calling
OS_Start(), the system stack is used only when no task is executed for the follow-
embOS software timers (and the callback).
For details regarding required size of your system stack, refer to the CPU & Compiler
Specifics manual of embOS documentation.