Measuring Spinnakers

[Jmolan]

I am looking into getting another used spinnaker. I am wondering how they are measured.

http://www.sailexchange.com/Spinnakers.pdf

At this site. They use Leech and mid-width. My question is, is the Leech measurement when the sail is pulled tight? If that is so, how do you get a good idea what size leech you need? My mast head is 41' above deck. Also mid-width must be how wide the sail is at mid point? Again is that pulled tight? Would a mid-width of 22' be 22' when it is hoisted and filled? Or something more or less?
I am looking for a very light weight "sky blotter" As I have a Trimaran, and I can take advantage of the 22' beam. My current Spinny is very narrow (less than the boat beam).
Thanks for the help ahead of time.

[Joli]

Sym kites are generally measured as:

.95 luff max = square root of (I squared + J squared)
SMW = 1.8*J