Yes, the Illuminatis do get bigger. I would say dime sized polyps is the norm for these, but I have had them bigger and have a frag now that has very small polyps. I have found the larger the colony gets, the healthier it is and is more able to withstand problems like parameter swings.
Strive to keep parameters stable, mainly salinity and nitrates. The newer your tank is, the more difficult it will be to keep nitrates and phosphates at the right levels for zoas. As for lighting, blues are what you are looking for, but you can add some whites for your viewing preference. Intensity of lighting is important also. I have my strongest PAR at 180, but there are areas down to 80 PAR also, so I can experiment with different frags as to where the sweet spot is.
Feeding definately helps, but not pellets. Get yourself one of the dry coral foods like Benepets, Reef Roids or Reef Chili and spot feed this per instructions as you don't have many frags. One of my favorites to feed is the LRS frozen food called Reef Frenzy. I chop this up really small and broadcast ffed my zoa tank.......they love it. They show way more feeding response than any other food that I feed. You should spot feed this though until you get a lot more coral.
Amino acids help too. Brands like Aquavitro FUEL and the Red Sea Reef Energy AB+ are proven coral growth additives.