If you're talking about static extenders then you won't get smaller than 18mm diameter. A G1/4 thread is 13.1mm major diameter, the bore is 10mm and then you need enough material to actual create a female thread. Alphacool and Barrow are the smallest I've seen at 18mm.
As for the actual tube fittings, that's all down to the type of tube and connection method. I'm working on a project that will use 12mm acrylic hardline in an In Win Chopin case and I'm using Barrow push-fit connectors, rather than compression fittings, because there is no space otherwise. They're 18mm diameter because there's no compression collar, just 3 o-rings inside a solid cylinder. I'm also using a couple of old-style 12mm tube EK-HD push-fit connectors for my current project where I just cannot get my fingers in to tighten compression collars and they're (I think) 20mm.
You can get weird things though. I did a fillport experiment for my current project using 10/6 soft tube and actually have a Bitspower 6mm barb for it that's still G1/4.
