2 layers15 × 27.5 mm

Paw3805-bb
efogtech/endgame-trackball-hardware
Because it's cheaper than 4 layers. Yes, seriously.
12 parts26
Boards in 1 GitHub repository, designed in KiCad, that use the RTT019533FTH by RALEC (-55℃~+125℃ 25V 50mW 953kΩ Thick Film Resistor ±1% ±200ppm/℃ 0201 Chip Resistor - Surface Mount ROHS). See how each wires it, with its parts, size, README and files; the RTT019533FTH’s symbol, footprint and pinout are on its own page.

efogtech/endgame-trackball-hardware
Because it's cheaper than 4 layers. Yes, seriously.