Replying to Ski Length Help

Looking to take advantage of online sales going on right now however I am learning ski companies measure their skis differently. Specifically if anyone can tell me how K2, Armada or Faction measure the length of their skis that would be a huge help. This is my first time ordering skis online and I’m not trying to get myself into anything that doesn’t size up correctly, so if the short answer is to go to the local shop I can live with that. Thanks in advance for any help!