I use calipers to measure the spar at the location of the fitting (with tapered spars). Main thing is to not go larger and have a loose fitting. In many, many cases fittings are stretched or enlarged to create a tight fit. Many builders drill out APAs to make a custom part. The "stock" sizes of each fitting are generally listed along with a letter code (C, D, etc)
Agreed. Match the connector plug in hole size (A, B, C, D) to the spreader end which is kite specific depending of the spreader tube used and how much of the tapered end is cut off.
The base spar hole is then reamed if needed to insure a nice tight friction fit on the leading edge tube. A tight installation can be aided by boiling the connector to heat and make the material more pliable or use 100% silicon so the connector slides easier.
If the connector is too loose on the leading edge tube, a couple wraps of clear tape will fill the gap and prevent sliding.
Lastly some builders grind off about half of the connector material around the leading edge tube to make a cleaner installation that reduces the leading edge bulge at the connector.