Anal capacity of a Zil is 44, twins get 10-15% bonus depending on the twin so it's ~50 tops. Vaginal capacity of a Zil female is 117, with 10% bonus it's ~128.
The catch is, for both twins the buttons are enabled based on their smallest (i.e. butt) capacity. So anything bigger than 48-50 is no go. 12 * 3 * 3 = 108 cubic inches.