The size difference is relatively straightforward. Depending on the phone you’ll be using, you’ll need to choose the appropriate size SIM. Standard SIM cards are the largest, followed by Micro, and Nano. You’ll find that a majority of SIM cards are designed to be compatible with all sizes. They come attached to a larger casing, and you pop-out or trim down the size to match what you need. That’s where the “3-in-1” moniker comes from. Provided you have a standard SIM, there are workarounds available to trim down the card so it matches a smaller size. You cannot go backward, as Nano-sized cards are much too small to fit in standard SIM sockets.