There are 3 sizes of SIM: 'Standard' which is the largest and now rarely used, 'Micro', which is smaller, most common SIM type, and 'Nano' which is the smallest.
If your phone is a recent UK phone (but not an iPhone), it will probably take a Micro SIM.
If your phone is an older model, you might need to use the SIM adapter we’ve provided. This plastic surround helps your new Micro SIM fit into the larger Standard SIM slot in older phones.
If you have an iPhone series 5 or above you’ll likely need a Nano SIM. This is smaller than a Micro SIM, but please don’t try to cut a Micro SIM down: it usually doesn’t work and we wouldn’t want you to be delayed in using your new phone.
If you're unsure which size SIM card your phone takes, take a look at the manufacturer's website to be sure.