A coupling that is as small as possible for my 3d printer.

There is a coupling and a hole. You substract the hole from the pieces you want to couple, then print a coupling, insert coupling to attach the pieces. Like this you can make overhangs, ... and have things nicely fitting, eg to try things, or to easily apply glue. Two examples of use are visible in the example, but this thing is the coupling, not those examples.

I need this to connect small pieces for another thing, that will come later!

I printed the coupling at layer hieght 0.25, 0 infill and 1 shell, as well as layer height 0.12, 50% infill and 0 shells, all with good results.

This thing is meant to be used in combination with other things. Eg, you want to print a human with overhanging arms. This mini coupling can be used to attach the arms to the torso.

There are two examples of use of the mini coupling here. One is coupling 1/8 of a torus, to create a combined thing, or adding a nose to a cork by adding a little nose and using the mini-coupling+pin, where the pin is used to push the nose in the corck