We are interested in how airlines make decisions on aircraft size and service frequency in a competitive environment. We apply three game-theoretic models to analyze airlines' choices in duopoly markets: one short-haul market and one long-haul market. We study how airlines' choices in a competitive environment may vary with flight distance, and also do sensitivity analysis to explore how the equilibrium results may change when air travel demand is higher, as it may happen in the future. Our research considers the competition factor in airlines' decisions on both aircraft size and service frequency, and the impact of these decisions on both the cost and demand sides of airlines' business. Different from previous studies, our research is based on cost, market share and demand models derived from empirical studies.