Turnbuckles are measured in the center portion only.  It is the "take up" portion of the turnbuckle.  The measurement is actually inside to inside of the two nuts at either end.

It is easy to tell if you have a 1/2" turnbuckle or a 5/8's inch turnbuckle because that measurement is actually stamped onto the turnbuckle itself.

Turnbuckles also have an R and an L stamped at the ends.  This represents the type of threading.  For example, the R means that end is "right hand threaded", while the L means that end is the "left hand thread".

If you have the R end attached to your ring post, you will turn the center portion of the turnbuckle to the RIGHT, in order to tighten it.  

If you are not certain of the size of your turnbuckle, 6", 9", 12", etc. you will measure the space INSIDE the center take up section, as shown below.  You do not measure end to end.  While a 6" turnbuckle may fully open up to be 18" long, it is the measurement in the center that determines the size of the turnbuckle.

See the below photos.

 How to measure a turnbuckle