The size of what a paly will eat depends on the paly. I have some that are larger & more cup shaped & they regularly close on full sized mysis & eat it. The flatter ones (white people eaters in this case) I feed small pieces of mysis & they close on it & eat it. We also have a powder form zoa/LPS food they love, but we don't feed it very often.
The zoas will sometimes eat the smaller powder food, otherwise they just get whatever goes their direction in the water column. They grow plenty fast without direct feeding, but I guess they get indirect from the thaw water (from thawing mysis cubes) that I squirt on the leathers. Good light & stable water parameters provide the best growth for zoas, IME. Once they get going, they'll go fast. It just takes months/a year before they get going.