The majority of the world still relies on the regular sized SIM card which measures in at 25x15x0.76mm. Not many phones use the microSIM, although the big names would be the iPhone 4, iPhone 4S and the 3G versions of the iPad and iPad 2, and recently Nokia started to employ the use of the microSIM as well. The microSIM measures in at 15x12x0.76mm, and if you thought that was tiny already, the nanoSIM will be 30% smaller at 12x9x0.646mm.
For those wondering about the significance of a smaller SIM card, what this means is that handset manufacturers will be able to fit more things into their phones. This could translate into much slimmer devices with a larger battery. According to the company, we will be able to see companies adopt the nanoSIM form factor as early as 2012.