That is, to scale a recipe up or down, hold the relationships of ingredients proportionate to that of flour, by mass.
E.g., a typical starter is fed at a 100% ratio: 1 unit of flour to one unit of water. So, 100g, 1,000g, 200g, whatevs. Similarly for your dough, which is often described in terms of hydration. A very low hydration might be 50%, a typical novice dough about 60--65% (excellent for pizza crusts and rolls in my experience), 80% should start giving a very open crumb (with large voids and bubbles), and 100%+ used for high whole-grain flour, or very open-crumb breads. Higher hydration makes for more challenging handling, though with time most bakers develop a feel for this.
There are of course a myriad of other factors, and one of the joys (or frustrations) of sourdough baking is exploring those parameters. My experience is that even my disasters taste amazing, though visual appearances may be less impressive. Amongst those: temperature (hugely significant for both starter and sponge/dough/proofing), proofing time (warm and cold/retarded), types of flour, age of flour, oven temperatures, humidity, and probably a whole lot more. Again, even when things don't go to plan, the results are almost always rewarding. When you do happen to hit the magic balance, it's amazing. My bad batches are still amongst the best bread I've ever eaten.
Good baking books and online guides will discuss this.
