Assuming you have a class applier for each font size, you could just iterate through each in turn and check if it is already applied to the current selection. It's not very efficient but may perform well enough.
var font16Applier = range.createCssClassApplier("font16");
var font17Applier = range.createCssClassApplier("font17");
var font18Applier = range.createCssClassApplier("font18");
var appliers = [font16Applier, font17Applier, font18Applier];
for (var i = 0, len = appliers.length; i < len; ++i) {
if (appliers[i].isAppliedToSelection()) {
if (i < len - 1) {
appliers[i].undoToSelection();
appliers[i + 1].applyToSelection();
break;
}
}
}