Hi,
You can actually change the font size base on device/screen width.

2. Screen/Device
To use this option, you first have to enable a Custom Grid Size for additional screens/devices.
Toggling between the different options here will allow you to style and position your Layer content based on the selected screen/device.
For example, if you had a text-based layer, you could assign it a font-size of 64px, with a top-right position for desktop view. Then if you switch to the smartphone view, you could change the layer’s font-size to 32px, and give it a bottom-left position.
For more information, kindly refer to the link below
https://www.themepunch.com/revslider-doc/slide-layers/#layercanvas