Important oversizing factors are observed for room air-conditioners leading to important energy wastes and costly summer peak demands for utilities. This article intends to show that sizing decisions should be based on dynamic simulation results and include an assessment of the air conditioner part-load performance. The sizing methodology established is applied to evaluate different oversizing levels for different types of buildings, climates and systems. This enables to compare our methodology to typical rules of the thumbs applied in France. By coupling these results to a stock model of room air conditioners, it enables to evaluate the potential benefits of better sizing in terms of energy.