A "fid length" is "21 diameters" of the rope you are working on.

eg. a fid made to work 1/2" rope by convention should be 10.5 inches
(21 X 0.5) long.
eg. a fid made to work 10mm rope by convention should be 210 mm (21 X
10) long

so a fid length is not the size of the fid you are working with (in
case you're working with a too short or too long a fid; but, 21 X the
DIAMETER of the rope.

In article eMlPf.485322$0l5.467600@dukeread06, Glenn Ashmore
wrote:

And whose fid? Most splicing instructions say "measure x fid lengths" but
seldom state it in inches or rope diameters. There are several makes and
different lengths of each but there are less fid sizes than there are rope
sizes so how would you define a "fid length"?