One interesting usage of sizeof() is to calculate the dimension of an array at compile time:

  #define dim(x)   (sizeof(x) / sizeof(x[0]))	

and this is handy for static arrays with dimension defined by its data, likes:

  static const struct {
    someEnum_t enCommand;
    void       (*Fxn)(stCommands_t *a_psCommand);
  } m_stCmdMap[] = {
    {CMD_SD_FREESPACE,         _sdFreeSpace},
    {CMD_SD_DISK_INFO,         _sdDiskInfo},
    {CMD_SD_FORMAT,            _sdFormat},
  }

When you add a new array elements, it just keep the stuffs going calculating it at any recompilation:

  for (i=0; i<dim(m_stCmdMap); i++) {
    if (m_stCmdMap[i].enCommand == stCmdLocal.enCommand) {
      m_stCmdMap[i].Fxn(&stCmdCopy);
      break;
    }
  }

Another useful macro to work with structs is offsetof(), its incredibly usefull when you have to port some code to different compilers/target with different alignments and paddings.