how to extract specific word from string in Postgres

Product name contains words deliminated by space. First word is size second in brand etc.

How to extract those words from string, e.q how to implement query like:

select
  id,       
  getwordnum( prodname,1 ) as size,
  getwordnum( prodname,2 ) as brand
  from products
where ({0} is null or getwordnum( prodname,1 )={0} ) and
    ({1} is null or getwordnum( prodname,2 )={1} )


create table product ( id char(20) primary key, prodname char(100) );

How to create getwordnum() function in Postgres or should some substring() or other function used directly in this query to improve speed ?