Start with your sit-bone width and riding position. A shop can take the measurement, or you can use corrugated cardboard at home, measuring between the centres of the two impressions. Repeat unclear readings and consult the manufacturer’s sizing guidance.
Before replacing the saddle, also check its height and position. Even a suitable model can feel uncomfortable when incorrectly placed; a bike fitter can assess your overall position. A friend’s favourite saddle will not automatically suit you.
Ask the shop about trial and return arrangements. A short ride on a properly adjusted bicycle will tell you more than the saddle’s appearance or squeezing it on a shelf.