how to recover the stack used by main(). with winavr gcc ?

It is not easy to do it automatically.

You can reuse the memory manually by setting up the linker script to give you information on where the stack starts and ends. You can use the linker variables directly from your C code.

Another thing that some ports do is use the main() stack as an interrupt stack, switching to the interrupt stack on interrupt entry and switching back on interrupt exit (a bit more complex if the port allows interrupt nesting). This then enables the stack allocated to each task to be smaller, so its a double win.