As technology have advanced, the sizes of some SIM cards have gone down aswell.
Here are the few different sizes of SIM cards you might come across:
Standard SIM (1st Form Factor)
The Standard SIM was the very first of its kind and the original form factor that was released for public use. These are not seen being used nowadays.
Mini SIM Card (2nd Form Factor)
The Mini SIM card is the second largest of the four. Mini SIMs are mainly used on older model phones and also very basic phones such as the Nokia 105. This SIM measures 25x15mm.
Micro SIM card (3rd Form Factor)
The Micro SIM card is essentially a Mini SIM card with the extra plastic around the circuit board trimmed off.
This SIM is more likely to be used for smartphones introduced from 2010 onwards such as the Galaxy S5, iPhone 4S etc. Micro SIM cards measure 15x12mm.
Nano SIM card (4th Form Factor)
The Nano SIM card is the most recent type of SIM card which only measures 12.3x8.8mm. It's used for newer smartphones, such as the Samsung Galaxy S9, the iPhone series from 5 onwards etc.
Important to note, all new Micro and Nano SIM cards are attached within a Regular SIM-sized unit which you can pull off or break off. This SIM is known as a Tri-SIM as you can use it for both. This is shown in the image below.