When choosing a size - find a pullover that fits the person you will knit for. Place this pullover flat on a table. Measure the width under the sleeves. That is what you need to know. The length can be adjusted (but if you make it longer you may want more yarn). Do also measure the length from sleeve-edge to sleeve-edge -  called wing-span. Deduct the width from the wing-span and divide by 2 to get the sleeve length you need.