Which func tells the tuple size of a given relation?

I am hacking the parser to do multi-class-classification. I am wondering
which function can tell the size of a relation. I tried to do something like

int row_sz = 0;
pgstat_count_heap_fetch(&rel->pgstat_info);
row_sz = ((PgStat_TableEntry
*)((&rel->pgstat_info)->tabentry))->t_tuples_fetched;

But the system does not like it and the DB broke down halfway.

Thanks,
John