Defining your base font size is an important step for responsive design. The base font size refers to the “default” or “primary” font size used in a website or app. The base font usually applies to most paragraphs, labels, menus, and lists.
Most modern UI and web utilizes a base font size of 16px. This is always a good default to start from as it is legible for users to read text on screen.
Ultimately, you want the body text on your phone (when held at a natural distance) to be as readable as the text in a well-printed book (when held at a natural – usually slightly farther – distance).