Size of polyp does not indicate whether it's a Paly or Zoanthid.
Paly's grow on sand and use absorb sand to use in their structure. Zoanthids do not absorb and use sand. That's really the best way to tell them apart.
like above ......check the stems of the polyp and see if there is grains of sand embeded in the stems ...if there is them its a paly.....if not them its a zoa