The corn starch will determine the thickness, so amounts for the liquids are needed.
The rule of thumb I use is 1 tablespoon of cornstarch to 1/4 c cold water. Combine the two until there are no lumps. Add to your sauce to thicken, however, the sauce has to reach a boiling point to thicken. If your sauce is not as thick as you like it then you can repeat the process. Cornstarch has twice the thickening power of flour.
I was always taught that generally speaking 1 tablespoon of cornstarch mixed with 1/4 c of water will thicken 1 cup of liquid and 2 tablespoons of flour mixed with and equal amount of butter or water will thicken 1 cup of liquid.
Temperature, humidity, the type of flour used and how long the roux has stood before it is added to the liquid can all effect how much the sauce or gravy will thicken, so you may have to adjust from there, by adding more roux if you want it thicker, or a little more liquid if it becomes too thick.