The size of SIM card you need depends entirely on your phone.
Very few modern phones use the standard size, so you’re probably going to need a micro or nano SIM. If you know the make and model of your phone, a quick Google search should tell you what size you need.
If you do have the wrong size SIM, there’s no need to fret.
A SIM that’s too small will fit into a bigger socket with the use of an inexpensive adapter (these can cost as little as £1).
A SIM that’s too big can be fixed with a pair of scissors by simply cutting away the extra plastic. If you’re confident, you can trim a SIM card down yourself by using instructions and templates which can be found online.
Otherwise, take your SIM card to your local phone shop where a professional can cut it down for you.