My smallest is one I made myself. Interestingly the knife was started as a prototype for SouthNarc, who I believe is afiliated with the Hideaway system, but don't quote me on that. I just finished one for a customer. I just realized I left my %$^! camera at home, so it'll be later today when I can send a pic, but one I just finished for a customer in the UK is 2 1/4" OAL (about 57mm) with a 1 3/8" blade (about 34mm). Steel is differentially heat treated 1084 (1/8" thick) with an etched blade to reveal the hamon. Handle scales are carbon fiber supposedly used in F-15 fighter jet wings. Pretty sweet little knife, and comes with a little tiny Concealex sheath suitable for neck or keychain carry. Pics will be here soon. I have two left, one with a drop point and one with a Persian/upswept point. Variety of handles from natural woods to synthetics of all types. Honestly, probably not a super practical design, but it is the type of knife you can stick on a keychain or something and it goes largely unnoticed. Email me if you want to talk prices, but I'd say this is more of a tool for someone highly trained in self-defense or someone who wants something unusual and cool on their keychain for doing little daily tasks.
As far as a really good, useful self-defense neck knife design, I think it's hard to beat the LaGriffe, either Fred's version or the Emerson version. Very secure grip in a pretty tiny, very flat package.
