programmatically retrieve details of a custom Postgres type

Hello,
I was unable to find how to get column names, sizes and types for a given
composite type.

Example. For a type defines as:
CREATE TYPE inventory_item AS (
name text,
supplier_id integer,
price numeric
);

I have a plpgsql stored proc that returns SETOF inventory_item (i.e. there
is no table with a column of this type).

I looked into the pg_type table but it only contains oid and typrelid for
the inventory_item type. I need a query that returns information about
structure of the composite type, i.e.:
ColumnName | ColumnType | ColumnSize
name | text | -1
supplier_id | integer | 4
price | numeric | 16

Is this possible? I'm executing queries via libpq...

Thank you!