如何计算从具有自动布局宽高比约束的xib加载的UICollectionViewCell的大小？

I have a UICollectionViewCell that is loaded from a xib and stored in a property to help with calculating cell sizes for my collection view. The collection view always has only one column, so the cells vary in their widths based on the phone the user has (iphone 5/6/6+). The xib has an image view with an aspect ratio that must remain constant for all phone sizes.I have a UICollectionViewCell that is loaded fr