What are the smallest microcontrollers?

I am looking for a microcontroller to use in a project which is very space-limited. What microcontrollers have some of the smallest packages? This does not need to be high performance, most microcontrollers are more powerful than is necessary in this case, but it does need to be tiny. Atmel's ATtiny20 comes in a WLCSP-12 package which is 1.55x1.4x0.53mm, this is not bad. What other options are there along those lines or smaller? (From any family, doesn't have to be Atmel)

Update: with at least 4 I/O pins.

Answer

Thanks to everyone who pitched in with suggestions, I have tried to collect all the info in the same answer.

The smallest microcontrollers from each family:

• Atmel ATtiny20-UUR: 1.55 x 1.40 x 0.53 mm, WLCSP-12, 10 gpio, 0.4mm pitch (the winner)


• (new) Cypress CY8C4014FNI-421: 1.56 x 1.45 x 0.4 mm, WLCSP-16, 12 gpio, 0.35mm pitch


• (new) Maxim MAX32660GWE: 1.55 x 1.57 x 0.64 mm, WLCSP-16, 10 GPIO, .35mm pitch


• (new) Freescale Kinetis KL03 MKL03Z32CAF4RTR: 2.00 x 1.61 x 0.56 mm, BGA-20, 18 gpio, 0.4mm pitch


• Freescale Kinetis KL02 MKL02Z32CAF4R: 2.00 x 1.95 X 0.56 mm, BGA-20, 18 gpio, 0.4mm pitch


• NXP LPC1102UK: 2.32 x 2.17 x 0.60 mm, WLCSP-16, 11 gpio, 0.5mm pitch


• Ambiq Apollo2 2.5 x 2.5 x 0.45 mm, WLCSP-49, 34 gpio, 0.35mm


• ST STM32F042T6Y6: 2.70 x 2.60 x 0.55 mm, WLCSP-36, 36 pins, 0.40mm pitch



• TI MSP430G2252: 2.65 x 2.55 x 0.28 mm, bare die, 22 pads


• TI MSP430G2x01: 4.00 x 4.00 x 0.90 mm, QFN-16, 16 pins, 0.65mm pitch


• Microchip PIC12LF1552T: 3.00 x 2.00 x 0.50 mm, QFN-8, 5 gpio, 0.5mm pitch

Sometimes there are other options in the same family which use the same package, but I haven't found anything with a smaller package. This doesn't take into account how much supporting circuitry each chip needs, of course.