I have this speed ring. Not sure which measurement you seek, but the interior diameter is 100mm, being about identical to that of the Profoto head. The outermost metallic diameter is 143mm, which is that portion being retained by the eight washers.
I sure hope it's the same: am about to upgrade my plastic SR. Just discovered the QR ring is available separately and I'm therefore able to salvage the Profoto insert (a part none too cheap).