Width: Choose a similar garment from your wardrobe that you are comfortable wearing. Lay it down on a flat surface and measure it from left to right across the chest, about 2cm below the arms.
This is the half chest measurement of the garment. Then using the sizing information for the garment you want to order, choose the size with the closest half chest measurement to your own garment.
Length: Measure from the highest point of the shoulder to the desired hemline.