Hmmmm, making a splice in 1/4 double braid is tough. I defy someone to do it with the smaller fid that comes with the NER unifid set...Too fat.
Allthumbs, I'll measure the diameter of the fid that I use for 1/4" double braid when I go to the shop tomorrow. But just going on my memory, I'd say that my fid is about 3/16".
I made my own fids about 12 years ago when I had had access to a lathe and a little too much time on my hands. So I honestly don't know what the diameters are.
As long as the fid you choose is smaller than the diameter of the core of the line that you are working with, it should work fine.
A nicely polished fid of the correct dimension will prove to be a most satisfactory tool.