Your sit bones, the bony protrusions that support carry your weight while sat on a saddle, vary in width from person to person. As such saddles come in a variety of width to accommodate. As such, to find your ideal saddle width you'll first need an idea of your sit bone width.
A good bike shop will feature a memory foam pad on which you can sit to get this measurement, but it is more than possible to get a good idea at home too. You can either ask a friend or partner extremely nicely if they'd mind getting the measuring tape out, or you can use the cardboard method.
Simply find a piece of corrugated cardboard, place it on a flat surface, and sit on it. Your sit bones should leave two small indents in the card; measure from the centre of these to get the width of your sit bones, and then add 25-35mm to give you a good approximation of your ideal saddle width.