postgres=# \d test
Table "public.test"
Column.     | Type    | Collation | Nullable | Default
------------+---------+-----------+----------+---------
sourcefile | text     |           |          |
sourceline | integer  |           |          |
seqno.     | integer  |           |.         |
name.      | text     |           |          |
setting.   | text     |           |          |
applied   | boolean   |           |          |
error     | text.     |           |          |
Indexes:
"test_idx" btree (sourcefile)
"test_idx2" btree (sourceline)

-- Find the table_size ( excluding the index_size)

postgres=# SELECT pg_size_pretty (pg_relation_size('test'));
pg_size_pretty
----------------
30 MB
(1 row)

-- Find the total_index size of the table

postgres=# sELECT pg_size_pretty ( pg_indexes_size('test'));
pg_size_pretty
----------------
26 MB
(1 row)

-- Find particular index size:

postgres=# select pg_size_pretty(pg_total_relation_size('test_idx'));
pg_size_pretty
----------------
19 MB

postgres=# select pg_size_pretty(pg_total_relation_size('test_idx2'));
pg_size_pretty
----------------
6496 kB

Another method:

postgres=# \di+ "test_idx"