The single flowered dahlias I am making in this tutorial use eight petals per flower and the single flowered dahlias are a scale three to four inches in diameter. To make single dahlias in your chosen dolls' house scale you will need teardrop shaped petals which are roughly 1 1/2 inches long in your chosen scale.
For 1:12 scale dahlias, I used a daisy paper punch with half inch diameter (12mm) daisies. You can also use a paper punch which punches individual teardrop shaped petals the correct size for your scale. Heart shapes cut in half through the lobe and the point may also give you petals the correct size. The daisy shaped punch is the easiest to use in terms of shaping and coloring the petals.