What are the smallest microcontrollers?

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.

ATtiny4, ATtiny5, ATtiny9 and ATtiny10 are packed in different 6pin packages some of them very small. They all have 4 I/O pins. Some of them ADC. Check for details on ATMEL site.

EDIT: After some more search. Well, ATtiny20 in WLCSP package is even smaller. So, it becomes the smallest available Atmel microcontroller. (only in ATtiny20-UUR variant)

The NXP LPC11A00 is an ARM Cortex-M0 controller that comes in a 20-pins WLCSP package:

Comparable in size to SOT-23, but those (AVR and PIC) only have 6 pins. ARM is also 32-bits.