The use of a suction dent puller really is dependent on the size of the cup attached to the puller. The larger one with the 6” cup can be used to pull up to 6 inch dings. Some of the dent pullers have a 4” cup, so this obviously will work on smaller areas. The secret to using this tool is to have a fairly smooth area where it will be used and whether the area is on top or on the side. It will not work on areas where the metal has been folded. If you require extra suction, all you need to do is wet it slightly and this gives a better grip to the metal.
For more info, see How to Repair an Auto Dent Yourself