Depends on what you want to use it for. For sitting on, or using for "sit-and-stand" squats, or for using as a support for your forward hand when doing one-arm lat rows, I like 18". For step-ups, I wouldn't go above 14". Rather than increase the height, I find it helpful to focus on step=up form and on squatting down a bit into the lower position before pushing up through the heel.