How long are the inlay blanks?
Most of the inlay blanks are sized for Sierra-style pen kits. . If not, the website should say what they are sized for.
I have found that the inlay blanks which are sized for Sierra-style will fit both the Sierra Twist (which has a brass tube about 2.2" long) and the Sierra Click (which has a brass tube a little longer than 2.4" ). . In both cases, you have to trim the inlay blank to the correct length after you glue the tube into it.
Sierra-style pen kits include the Mesa, Gatsby, Zodiac, Ellipse, Wall Street, and other kits from various manufacturers. . Those all take a 27/64" diameter tube.
Other kits which the Sierra-style inlay blanks will fit are the Virage, the Napoleon, and the Le Roi Elegant Major. These also take a 27/64" diameter tube.
Another type of pen kit for which you quite commonly see inlay blanks is the PSI 30 Caliber Bolt Action kit which is designed around a 3/8" diameter tube.
Hope this helps.
Note that when we pen-turners refer to a certain diameter of pen tube we are really talking about the drill size we use to drill the hole in the blank to accept that tube. . The tube is actually about 0.010" smaller than the drill.
Inlay blanks, as stated above, are pre-drilled ... you don't have to do any drilling.