I would like to suggest Plus-/ Minus-Buttons for the Frontend to increase/ decrease values like Font-Size etc. Either as an addon to the slider-/ inputfield-element or maybe even as a standalone replacement for the slider. In my opinion it is more useful if a customer have to handle just small range-adjustments (eg. Font-Size-Adjustments between 10 and 20). Also i have noticed that especially elder people prefer this method more then a slider.
In a deluxe version, the admin could choose how many incremental steps should be made, if a user pushes the button.