Simplest way is to use dimensions in % or em. Just change the base font size everything will change.
Less
@media (max-width: @screen-xs) {
body{font-size: 10px;}
}
@media (max-width: @screen-sm) {
body{font-size: 14px;}
}
h5{
font-size: 1.4rem;
}
Look at all the ways at https://stackoverflow.com/a/21981859/406659
You could use viewport units (vh,vw...) but they dont work on Android < 4.4