The chapter lists out and explains several categories of dimensional and unitless aircraft design parameters. The chapter also presents a methodology for estimation of these parameters. In particular, the reader will learn how to determine the design wing loading and thrust loading, the aircraft gross mass using the unity equation (in two iterations), as well as the aircraft mass breakdown. The chapter presents some historical data on mass ratios of different components of various classes of aircraft. The chapter explains the Breguet equation for calculating aircraft range and shows how it can be used to estimate the required fuel capacity for a thermal aircraft. Finally, the chapter exemplifies the above calculations for a mid-range commercial airliner.