If oversized object penetrating the character's orifices, the narrator indicates the dimensions of this object in the metric system (centimeters, millimeters). Length, diameter or circumference, and shape. This measurements is not necessary for items whose size is known to the reader. Instead of specifying dimensions, you can compare an object with something else. Further, when the object penetrates the character, the narrator, when appropriate, reports the depth of penetration also in the metric system.