It is not obvious, and requires some experience with ‘handle diving’ into graphics objects.
Try this:
H = tf([2 5 1],[1 3 5]); % Define System
pzmap(H) % Create Plot
hm = findobj(gca, 'Type', 'Line'); % Handle To 'Line' Objects
hm(2).MarkerSize = 20; % ‘Zero’ Marker
hm(3).MarkerSize = 20; % ‘Pole’ Marker
This changes them from their default size of 7 to 20.
EDIT — This works in R2018b. I cannot test it with earlier releases.
