ATtiny20 is available WLCSP (Wafer level chip scale package) form factor, which is the bare chip with absolute minimum of padding required to mount it.
The summary datasheet on page 12 specifies for the WLCSP:
1.540 x 1.388mm Body, 0.433 mm thick.
Width and length will be exact match, but thickness includes the pads. A technical drawing on page 13 gives the chip thickness, although with tolerances:
min: 0.228mm, nom: 0.305, max: 0.330.
Assuming nominal values then, 1.540 x 1.388 x 0.305 mm