pg_table_size('index_name') for individual index – but it only shows you the size on disk, not the number of entries.
count(*) to get the exact current number of rows
sum(pg_column_size(column_name)) from table_name for estimations on column data size.
You can try something like:
t=# \di+ tbl*
List of relations
Schema | Name | Type | Owner | Table | Size | Description
--------+-----------+-------+----------+-------+--------+-------------
public | tbl_pkey | index | postgres | tbl | 156 MB |
public | tbl_unpic | index | postgres | tbl | 46 MB |
public | tbl_x1 | index | postgres | tbl | 57 MB |
(3 rows)
t=# \dt+ tbl
List of relations
Schema | Name | Type | Owner | Size | Description
--------+------+-------+----------+-------+-------------
public | tbl | table | postgres | 78 MB |
(1 row)
t=# select pg_size_pretty(pg_total_relation_size('tbl'));
pg_size_pretty
----------------
337 MB
(1 row)
t=# select 78+57+46+156;
?column?
----------
337
(1 row)
and to check how psql gets the individual index size, run it with psql -E.
and once again – functions above work with size it takes on disk – it may/(may not) be extremely different from real amount of data. vacuuming helps here
update
I don’t know where you directly get the number of “rows” in an index, thus I can only offer indirect way. Eg let me have a partial index, so the “number of rows” in the index is different from the number of rows in the table. I can check estimations with EXPLAIN (of course you have to repeat the WHERE clause for that) checking the rows=66800 in Index Only Scan using gives me an idea on the number of rows in that index (actually it is rows=64910 which you can get by explain analyze or just running count(*)). I can’t find relevant info in pg_stats – maybe there’s some formula. I don’t know.