why is it klein and not kleine? I know that's what she said but shouldn't the plural be with -e ending?
If it's not in front of the noun it's referring to it's always without -e. So: Grote kranten zijn mooi.
and what does "Grote krante zijn mooi" mean? "Big newspapers are..."?
'Klein' is a predicate adjective (see Tips and Notes), i.e. it follows the linking verb 'to be' (zijn).
Predicate adjectives don't get any ending.