Recorded male Hercules beetles reach roughly 17 centimetres from the tip of the thoracic horn to the end of the elytra. The body alone is far shorter, often under half the total, because the horn is an elongated outgrowth of the pronotum rather than part of the abdomen or thorax proper.
The horn is a sexually selected weapon. A male grips a rival between the downcurved thoracic horn and the upcurved head horn, lifts him, and attempts to throw or crush him. Horn length scales steeply with body size, so large males carry disproportionately large weapons and small males carry almost none. Females are hornless and are usually the heavier-bodied sex relative to their length.
What the length record does not settle is which beetle is largest overall. By mass and by body length excluding horns, other species such as the goliath beetles and the titan beetle rank higher, so "largest beetle" depends entirely on the measurement chosen.