The definitive characteristic (as I understand it) that separates the two is whether ot not they take sand into their coenenchyme (stalks/mats, etc.). If they take sand and other grit materials, they are Palythoa sp. If they do not, they are Zoanthus sp. (or potentially one of the other critters that fall into the zoa category). Size doesn't matter at all, but Paly's tend to be larger than the typical Zoathus (the typical zoa being maybe eraser or jumbo eraser head size, while a typicaly paly may be a dime's diameter). PE's (PPE's, RPE's, GPE's, etc.) are Zoanthus gigantus and they are larger than most Paly's...