The Galaxy S6 (and the Galaxy S6 Edge), use the smallest SIM commonly available: a Nano SIM. This size, popularised by Apple with the iPhone 5, is a little smaller than the micro SIM and much smaller than the full-size SIM card.
If you have a micro SIM, you can cut this down using a Nano SIM cutter tool. Just slide in your SIM into the marked area, push down on the lever, and your newly cut Nano SIM will emerge.
You can always use an adapter if you need to use your Nano SIM in a micro SIM phone in the future.
You can also order Nano SIMs from all major mobile operators in the UK, although you may need to arrange a SIM swap with your operator if you want to keep your existing number.
I hope this has answered your question. If you’d like more information or to offer feedback, please comment below or tweet to us @mobilefun.