Use a tape measure to measure both the length and the width of the room.
Multiply the length of the room times the width of the room to calculate the room's total square footage.
Determine the cubic inch per minute (CMF) value for the room you will be installing the new duct in. The basic rule of thumb is that you want 1 CMF of air velocity per square foot in the room you're going to be cooling.
Use an online duct calculator to determine the correct duct size for the room you are going to cool. There are a wide variety of duct calculators to choose from online.