Not sure of Z, however...
N - can be as small as 9" radius, however anything under 11" is too tight in my opinion. Also, if you are planning on running the long passeenger cars, 19" minimum will look better.
HO - can be as small as 18" radius, however anything under 24" is too tight in my opinion. Also, if you are planning on running the long passeenger cars, 36" minimum will look better.