MPI_FILE_SET_SIZE is a collective routine used to resize a file. It is important to remember that a MPI-IO routine being collective does not imply that the routine synchronizes the calling processes in any way (unless this is specified explicitly).
As of 1.2.4, ROMIO implements MPI_FILE_SET_SIZE by calling ftruncate from all processes. Since different processes may call the function at different times, it means that unless external synchronization is used, a resize operation mixed in with writes or reads could have unexpected results.
In short, if synchronization after a set size is needed, the user should add a barrier or similar operation to ensure the set size has completed.