After reading a few related questions and the official tutorial, I have a guess, hoping to confirm:
If I do not need to add extra events into schedular for management purposes, only the default event (such as ble_evt_handler's event in main.c) is required. Can I set EVENT_SIZE in APP_SCHED_INIT to 0 because I looked at the SD handler's code and No put event into schedular, put only a function pointer
void SD_EVT_IRQHandler (void)
{
ret_code_t ret_code = app_sched_event_put (NULL, 0, appsh_events_poll);
APP_ERROR_CHECK (ret_code);
}