Measuring the size of a distant object using a gauge sized according to a range to the object is provided. A user may select an initial target scale size and use an aiming mark to align a rangefinder with the object before determining the range. Indicia may be associated with the gauge, both of which could be displayed in a field of view of an optical sighting device, such as a rangefinder, riflescope, or spotting scope. In addition, the gauge or the indicia, or both may be alterable in response to the determined range. While looking through an eyepiece of the optical sighting device, the user may align the gauge with the object and estimate its size by visually comparing the gauge and object with reference to the indicia. The gauge may be geometrically shaped and may contain a series of spaced apart measuring marks having a logarithmic spacing.