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)
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
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
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.
You can also obtain microcontrollers as dice - the actual semiconductor after cutting from the silicon wafer, but before packaging. These can be used as Chip On Board (COB) assemblies and can be much, much smaller than a packaged part.
Some companies also offer Chip Scale Packaging (CSP), which is only slightly bigger than that actual silicon dice inside the package. These are often under 0.5mm high, and between 1mm and 2mm in length and width.