How can I change the font size in the Dawn theme?

Store Design
2

You are right. It would be either h1 or h2. This is at the Asset folder, under the base.css. FInd the code .h1, h1 { } or .h2, h2 {} with a font-size. You can adjust the font by reducing the rem. NOTE: Dawn Theme uses rem, instead of pixels. It means it rely to the root font size, which is in the theme.liquid file, under the layout folder. The main root of the sizes came from html, which has a font size in percentage. I believe in Dawn theme, their 1 rem is equal to 10px. Let me know if that helps