It is rare for an engagement ring to be the correct size the first time, the nature of engagement rings tends towards secrecy, and you cannot very well ask your significant other their ring finger size without arousing suspicion.
Therefore it is sometimes necessary for a ring to be resized to ensure a comfortable fit so that it can be worn for years to come. Most jewelers will give you a free ring sizing gauge that can be used to assess the size of a finger (it looks like a small lasso). In addition to this many jewelers stock example rings which are a set size, and may ask you to try on various sizes and therefore give you a bit more say in how comfortable the ring will be. The general rule of thumb is that the ring should go on the finger easily and require a slight pull to get off, however, at no point should the ring feel uncomfortably tight.
Below we have a video that shows how a ring is resized and should hopefully help clear up any further questions arising about the resizing of a ring.