I own a few pairs of Cubics circs. You can measure them in a normal needle sizer. In this case, you can fit a square into a round!
The 'corner' points of the Cubics determine the diameter of the needle. The stitches will 'knit to' the size of the needle which is determined by those 4 corners.
Think of it as a US8 round needle that has been 'planed' to make it US8 square. The extra (shaved off) wood is in the the wastebasket.
Here is a photo of my US8 Cubics circ. It fits perfectly into the US8 hole of my needle sizer. Conversely, it won't fit into the US7 hole past the tip at all. And it is way too small for the US9 hole.
I constantly have to measure my KnitPicks nickel plated needle tips to be sure of size, and same goes for the Harmony needle tips. It's just one of the little annoying things about KP needle tips.
On the other hand...the Addi Clicks tips do have the size written on them...but it becomes so faint over use and time...that you have to measure them in the needle sizer as well. Gah.