Not exactly clear which measurements you would like. I am assuming the inner diameter of the coil for each size?
18- 26mm, 20-28mm, 22-30mm, 24-32mm, 26-34mm, 28-36mm, 30-38mm, 32-40mm, 34-42mm, 36-44mm, 38-46mm, 40-48mm.
The end caps are solid so they have no inside diameter so I assume you mean the I.D of the rubber tube and O.D of the end cap?… if so, it is about a 3.85mm for the insert section of the end cap into the 3mm open ID of the tube after heat shrinking.
The Corkscrews are slightly oversized when fitted over Innerwear of the same size. In which case, you could go for a size smaller if you want a snugger fit from the get go, which… if needed, can be screwed down into place on the shaft as if threading a nut over a bolt. One would then need less tightening of the coils when twisting down to make a tighter fit.
However, the smaller sizes such as the 18-20-22 girths have less latitude than say the larger 36-38-40 girth in which 2mm diameter or 1/4″ in circumference makes relatively less difference… whereas, the smaller sizes 2mm is more significant.