Keep the input text legible
The input text size, as well as the placeholder size, should be easy to read. There’s no single “right” size for text inputs, but 16px is a good starting point. It can be larger if you have a text-heavy page or slightly smaller if you design an interaction-heavy page.[1]
Pro Tip: Keep in mind that the legibility of different fonts can vary even at the exact same font size.
References
- A Reference Guide For Typography In Mobile Web Design — Smashing Magazine | Smashing Magazine

