There's some confusion over how the devices are measured and how they should fit.
The area in the inner cone is 10mm smaller then the device measurement
example: 35mm car-1 the inner cone hollow area is 25mm
if you are in the 35mm car-1 size it'll be just the tip and nothing more.
The area in the inner cone is about 5mm smaller than the device measurement. the retainer shout cover about 1/2 to 2/3 of the glans. the retainers are pretty forgiving on sizing.
Using my tape measure / sizing guide will point you in the right direction.