Likely the smallest Arduino-compatible with USB-C at the moment. It was originally intended as a controller for DIY keyboards, but it can be used for anything. The size is 22,75x12,75,3,25mm.

Has quite a few neat features:
Mid-mounted USB-C connector and castellated pins(quite obvious)
ESD protection on the USB power and data lines
Ferrite bead and polyfuse on the USB power line
Low-pass filter for the analog reference
Mounting slots
24 available IO, a couple more than most 32U4 boards

You may think the port would snap quite easily, but surprisingly it doesn't. I can't even break it off by hand.

The pin spacing is 1,27mm as you may've noticed, and it doesn't have any voltage regulator.

Also Crowd Supply picked it up, which is fun. Hopefully i'm allowed to mention that here.

Released under CERN-OHL-P.