The principles of biomass mechanical size reduction and models predicting specific energy requirements of biomass comminution on particle size characteristics and machine variables were summarized in this mini-review. It was identified that lignocellulosic biomass up to 25 wt % in moisture is usually mechanically reduced in size by cutting, shearing, tearing, or breaking, primarily provided by knife or hammer mill. The specific energy requirement of these size reduction machines depends on targeted size reduction ratio (initial over final characteristic particle size), on biomass properties (chemical composition, mechanical properties, moisture), and mill variables (geometrical set-up of working tools, rotor revolutions, drum screen sieve size). Specific energy demand for mechanical biomass size reduction usually ranges in units or low decimals of kWh t-1. Rittinger's comminution law was found to precisely predict a model determining specific energy requirements related to change in particle size characteristics of a hammer- or knife-milled biomass. Rittinger's constant varies with biomass properties and machine variables.