I echo the sentiments of
nomadmax and
Carbonfiberboy... shape matters. Your riding position also matters. Generally speaking, your saddle width will be narrower as your riding position gets more aggressive and wider as your riding position becomes more upright.
My sit bones are approximately 10.5cm, so theoretically a saddle as narrow as ~125mm
should fit me. However, in a "touring" position I've ridden a 155mm Ergon saddle that felt like my butt was going to swallow it (very uncomfortable to say the least) and I own a 157mm Gilles Berthoud Aravis that fits my booty like a glove (it's perfect). On a road bike with endurance geometry a saddle width of 146mm is excellent for me. Anything less than 140mm would likely start to become uncomfortable unless I was on an extremely agressive race bike.
At the end of the day, even if you know your sit bone width and can thus estimate proper saddle width for
you, you still need to ride the saddle to know if it will work or not. Once you find that correct combination of width and shape it becomes much easier to experiment going forward.