The bushing is solid with just a taper on the barrel end to seat the centers in.
Barrel diameter is 0.335" or 8.5mm
tube section diameter is 0.246" or 6.25mm
overall length 0.875" or 22.2mm
Length of tube section 0.492" or 12.5mm
I make my own TBC bushings and for the Slimline there are only 3 critical dimensions, the two highlighted above and the 60* recesses in the drive ends of the bushings. I find that the longer the "Length of tube section" is the more stable the overall setup is and of course this effects the "overall length". I actually pay very little attention the overall length. I have one set that the OAL is probably 1.5"
. I tend to make mine so that the "drive section" containing the 60* drive recess is about 1/2" in diameter and 3/8" to 1/2" long. That "tapers" down to the 0.335 dim which is maybe 3/8". That then "steps" down to the 0.246" dimension which is usually at least 3/4" long. If you add up all the lengths you will see that my SL bushings are pushing 2" and you could do them in 1" if you wanted to so don't get hung up on the length dimensions just make sure the two diameters and the 60* are dead on.
All that being said always verify the slimline parts dimensions, you may find that you have to turn the blank slight plus of the bushings or slightly negative of them. This caveat applies to all styles of pens and their bushings.
Edit: Just noticed that this thread has a little age on it but that doesn't change my comments.