1. Human thumbs are not as accurate as a computer mouse. Therefore, your call to action should be at least 44 X 44 pixels.
2. Font size on most smart phones should be 13 pixels.
3. Be bold. Use color, images, and typography to be visually compelling and engaging.
4. Because the screen size is small, keep the messages simple and concise.
Source: Fernando Espejel, “Send Smart,” Target Marketing, 6/13, p. 13.