Front derailleurs are measured in terms of the seat-tube diameter. Easiest way to measure is with calipers. You can do it with a piece of string as well (i.e., wrap a piece of string around the seat tube, measure its length, and divide by 3.14). Some manufacturers will publish seat tube diameters on their website as well; can't hurt to look.
Shifting performance is very rarely determined by the "quality" of the derailleurs. The main difference in derailleurs is the weight. Front derailleurs especially are simply two parallel pieces of metal. That holds true for the cheap ones to the expensive ones.
You can get lower end components to shift perfectly well. Keeping the drivetrain clean and well lubed goes a long ways towards this. Treating your bike to some shiny new well routed cables goes a long ways as well.