The K-11/12 mirror is over 3 metres[1]. The KH-8 had a resolution of 8-10 cm in the 1960s[2].

[1] http://www.globalsecurity.org/space/systems/kh-12.htm

[2] https://books.google.com.au/books?id=e1vSUjuEbHYC&pg=PA78&lp...