Eighteen millimetres square: the microcontroller, two RGB LEDs, the switch and eight tiny passives on the front; two chain connectors and a row of five programming pads on the back. Whatever goes on one board goes on all of them, so I kept it lean. The I²C pull-ups live once on the controller, not on every button.
The design check taught me a few things. Traces 0.25 mm wide don't fit between pins 0.4 mm apart, so I went to 0.2 mm. And one stray loop of copper had quietly cut a ground pin off from everything else; I only found it by hunting for unconnected islands.
The first quote was €90–100; the real total came to about €40, since almost all of the assembly cost is a fixed setup fee. Ten boards arrived, and every LED channel and every switch worked on every one of them. No bodge wires, no rework. A first revision almost never goes like that.