Sell, donate, and whittle down. Then get out a ruler.
Start with what's hanging. Place clothes flat, on their hangers, in two piles—one for short items, like shirts, and the other for long items, like coats and pants hung full length by their cuffs or waistband. Measure the height of each pile to get the desired lengths for short- and long-item rods. Keep in mind that hangers need sliding space.
Next up: Clothes that fold. Arrange foldables in 10-inch-high stacks—any higher and they could topple. Each stack needs 14 inches of shelf length.
Extras. Size up items like ties and T-shirts that also need real estate.