How do I determine which size is best for me? I checked Easton’s site which is no help at all as even the installation manual is for the old EC90. Anyone have suggestions as I believe they come in S, M, and L. Thanks.
Per the Excel Sports website (where I got mine) small is a 225mm extension length, med is 250, and large is 275. Note that these bars may not work for you i you are very tall, I use a Large and I’m only 5’10"
What is the measurement from your elbow to wrist joint?
Yep…ditto Jason’s comment. These bars run small…if you do a search on the bar her on the forum, you’ll see tons of comment echoing this view. I am 5’7" and I use a size medium and could have probably even used a L. The size S is very small (short aero extensions). If possible, I’d try to get my hands on one…if that isn’t an option, stay away from the S!! 
Where are those S, M, L measurements taken from? End of the aero extensions to what?
The bar is measured from the centre of the handlebar clamp to the end of the extensions. To answer your other question, my forearm measures approximately 280mm from elbow tip to the crease on my wrist joint. Add another 60mm or so if you want to measure from elbow tip to the centre of a dowel held perpendicular to my forearm (neutral forearm, i.e. neither pronated nor supinated).