Man pages matching name mpi_file_get_size in section 3:

MPI_File_get_size.3.en	debian 10 / lam-mpidoc
MPI_File_get_size.3.en	debian 10 / mpich-doc
MPI_File_get_size.3.en	debian 11 / lam-mpidoc
MPI_File_get_size.3.en	debian 12 / lam-mpidoc
MPI_File_get_size.3.en	debian 12 / mpich-doc