A 36 inch half slip is typically the longest slip on the market. Measuring about 36" from waist to hem, it will hit most women somewhere from low-calf to ankle. Unless you're wearing a floor-length dress or gown, it may be too long, so it's a good idea to measure exactly where 36" falls on your body.
There are many styles of long half slips to select from. Choose one with a high slit if your outfit also has a slit. A slip with a slit that can be worn in the back, on the side or in the front will be the most versatile. For knits or other clingy material, look for a microfiber slip that will gently hug your body and create a smooth unbroken silhouette. For fuller skirts, a silky nylon a-line or flared 36 inch slip is ideal.