This study investigates a sizing tool for the preliminary design of an Unmanned Aerial Vehicle (UAV). The UAV is designed with the aim to join and compete in the SAE Aero Design Student Competition. The sizing tool was developed in an excel spreadsheet to calculate the general dimensions of the UAV. Different designs were generated and calculated using a spreadsheet which was created to save time and effort in doing the calculations by hand. This sizing tool was used by different groups of students following the rules and regulations given by the SAE Competition rules 2018 document. The micro class aircraft must fit in a rectangular box with specific dimension in a disassembled condition. Moreover, the micro-class criteria competition is scored based on which aircraft has the highest payload fraction while simultaneously the lowest empty weight possible. In this scope, multiple alternative designs for the micro aircraft were created using the dimensions obtained from sizing spreadsheet/tool. The sizing tool helped the students to explore design space with selected input and output parameters, much faster and more reliable than individual hand calculations.