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As per AMS brochure, the cylinder length determined at the time of incision, not before. Depending on the result of widening the corpora cavernosa:

To implant the cylinders, your doctor will first dilate (widen) the corpora
cavernosa (the two channels in the shaft of your penis which fill with blood
when you get a natural erection). Then your doctor will measure this area to
choose which size cylinder will best fit your anatomy.

http://www.americanmedicalsystems.com/DAM_IFU/5616.pdf
Page 20

That sucks since one will never know before the surgery whether or not his corpora cavernosa is big enough to fit cylinders that will not result in too much of shortening. I am wondering if there is any kind of test prior to the surgery to determine the size of the corpora cavernosa; and if it is technically too small, then just give up on the surgery.

I was measured by the doctor's assistant stretching my penis and measuring its length. This is where repetition and a good doctor will help confidence in the procedure.

Is there any correlation between the flaccid size and the cylinder size? Or it is purely based on how much the CC can expand?
The thing is that I am of those who has a mini-dick while flaccid (around 3" long and very narrow) while erected is slightly more than 6" which is more than double. That means my CC is "very expandable" but I am wondering if during the flaccid state (during the surgery) that expandability can be accurately measured?