Zoas don't get bigger per se, they grow more polyps. When you "see growth" it will be in the form of another polyp.
Some grow like weeds and sprout new polyps seemingly every other day, some might grow a new one once every few months. That's why some are cheap and some cost $300 per polyp. ;)
It also depends on water quality, lighting, etc. Every tank is different.
A buddy of mine sponsored a grow-out contest. He gave a free single polyp frag from the same mother colony to 30 different people. We check in each month to compare growth. Some people's frags melted in the first week, some are growing like crazy.
As long as it's open and colored up, just be patient. I have 32 different zoas in my tank and they all grow at different rates.