The SIM card comes in different sizes: standard-sized SIM, micro-SIM, and nano-SIM card. One SIM card can be broken down into different sizes depending on what the phone or device is using.
The standard-sized SIM cards measures 15 x 25mm, which is shrunken down from the SIM card's original credit card size. While micro-SIM card is a smaller version of a mini-SIM and are popularly known to be used in the original iPad, GSM-compatible version of iPad 2 and iPad 3 and 4 generations. Lastly, nano-SIM card is both smaller and approximately 15% thinner than the mini and micro-SIM card and is commonly used in cellular capable iPad mini models and iPhone 5 models.
Refer to the picture below on how these SIM cards looked like.
For the devices that we sell which use a SIM card like iTrack and iTrack 2, they use the standard SIM card with prepaid (pay as you go) plan. The SIM card for these devices cannot be broken down into smaller form or the device won't work.