If you don't have a scale, go for 1/4 cup starter to 1/2 cup of flour to 1/4 cup water.
I keep 1 ounce of starter and feed it with 1 ounce of flour and 1 ounce of water because I rarely need a massive amount of starter on hand, but various other sourdough luminaries advise different amounts. If you don't have a scale, go for 1/4 cup starter to 1/2 cup of flour to 1/4 cup water. (Flour weighs less per volume than the water or starter.) I've found that for maintenance, it's fine to just have a small amount, and since starter is infinite, you can always feed it more if you need more starter. (The famous-among-bread-people Tartine country loaf recipe, for instance, only needs 1 tablespoon of starter for the whole loaf.)