Today I attempted (with success) to find out what is the optimal internal diameter of QLBRD. As it's supposed to fit as a bracelet there's a hard limit set by average wrist size. After measuring my own wrist and few others I found out that 6.5 cm is around the size that should fit in all cases (without right end of Gauss curve ofc). 

As I wasn't sure about actual feel and ability to spread enough while passing a palm I prepared two samples (6.5 and 7.7mm in diameter) of internal frame ring. These samples aren't actual ring, they are only cross sections of rough models. They measure 1/6th of height of actual ring. One feature worth mentioning is a split on one of protrusions. It will host small rare earth magnets that will act as a clasp.

One other dimension to note - 1.2mm for ring wall. Smallest meeting two conditions: to be durable enough and flexible at the same time. Smaller values tend to be very fragile when it comes to 3d printing.

One last thing to mention before showing photos - the ring is here for two main purposes. It gives QLBRD its spherical shape (not quite spherical, rather distorted toroidal...) and provides common power data lines (on kapton based PCB).
Tested both diameters on mine and others hands (however post only mine, not for sensitive eyes). Both fit and flex without a hassle. The bigger one is just too big to my taste. Later in process I'll prepare two or more size versions, but for now I'll go with the smaller, 6.5mm one.

The bigger one

Passes well

But little too big as the ball will be 12mm bigger than that.

Smaller one, less space inside the ball, but far more comfortable to hold.

As can be seen here.

Spreads wider and deforms more but in acceptable range (or predictions as to said range).

Leaves still a lot of space for free movement, doesn't compress hand from any side.