Fusible link is nothing more than a different way of saying inline fuse. The wire size and length are figured based on the amount of energy it will require to melt the link. The smaller wire means higher resistance so the link will burn before the higher gauge main wire, the length adjusts the total thermal mass of the element and it's total resistance, so it can be tailored to slowblow/fastblow based on length. It's no different than any other fuse, simply it's location is different.
Keep in mind you can't just use normal wire for the fusible portion of the link, if it ever actually trips that means it will have reached the metals melting point and one end will still be connected to 12V, which will have a piece of copper at it's melting point which can easily fuse to the chassis of the car causing a true dead short condition which would vaporize the rest of the wire harness to the battery.