When we talk about saddle size, that is mostly related to saddle width. Road bike saddles come in a range of widths, from approximately 125 mm to 160 mm (give or take). If you already have a saddle that works well for you, it may be as simple as getting one that is roughly the same width.
That said, getting your sit bones measured is the best way to zero in on the correct size to suit your anatomy. Most shops have sit bone measurement tools, and often they have test saddles that you can try out.