Hi Jiang,
the difference between iPhone and Android devices is in its screen resolution - in iPhone is fixed (all iPhone 6 have same resolution) and in Android it is different on each device.
what you can make is to use media queries in your CSS which will distinguish the device and apply different sizes..
google: "css media queries iphone ipad android"
Media Queries for Standard Devices | CSS-Tricks
also, you can try to use em as measure - it is not absolute like px but it is relative to the set font size.
google: "css px vs em"
http://stackoverflow.com/questions/609517/why-em-instead-of-px
Karol