I'll give you the Israeli ministry of transportation definitions:
סירה - עד 7 מטר
ספינה - 7-24 מטר
אניה - מעל 24 מטר
So a 36-foot yacht is definitely a ספינה and an oil tanker is an אניה. In common usage people will extend ספינה to bigger ships, but nobody will call the 36-foot yacht an אניה. They might sometimes call it a סירה, but that will get them angry stares from yachtsmen.
Ship was rejected. What is the difference between ship and boat, and what is the difference between סירה and ספינה ?