Well... that did not go well.
On the day of the keynote I printed the measuring form, verified the size with a credit card (perfect) and carefully cut out the measuring device. I literally taped the "watch face" to my arm, in the EXACT spot I wear my Apple Watch (I have a big tan line) and then wrapped the paper around my arm leaving NO GAP with my skin at all. The number in between the arrows was right in the middle of the 6 range. If I pulled the paper tighter so it would almost rip but created a big uncomfortable dent in my skin I could get it to read in the low 5s. But bottom line is with ZERO gap on my skin it was a 6... so I ordered a solo loop in 6.
It is HUGE. So loose its like a oversized bracelet so the watch will easily slide up and down several inches from the wrist/hand to further up my arm with every movement. There is visible air gap around the entire band and my skin. Clearly I likely need at least 2 sizes down, a 5 is still going to be loose is what I guess but my wife has a 5 coming in a few days (it is sadly obviously going to be too big for her also, we both have small wrists but hers are really tiny) so at least I can try that.
Frustrating experience, I'm returning it obviously and reluctant to order one again until I know for sure what the proper size hopefully by trying one on in a store... just not sure I want to make a trip to an Apple Store just for a silicone band for a watch.
For everyone's reference I have a left wrist of 164mm, 6.46 inches, and I normally wear an Apple Sports Band in 44mm, S/M size, 3 or 4th most tight hole depending how snug I want it... neither allow the watch to move from the wrist position I like.