How to crop a photo in inches on iPad?
The options presented in the crop dialog in the Photos app are ratios, and can be scaled to any size.

Since digital photographs are simply made of pixels, it is impossible to assign physical measurements to the somewhat abstract representation of an image. The physical size would depend entirely on what scale it is displayed or printed at. An image can have a pixel density of any number of pixels per square inch. For instance, even if you restrict the parameters to iPads with Retina displays, one square inch can hold 264 or 326 pixels.

If you're cropping an image so it will fit on a certain type of photo paper, you can use the ratio of width to height to determine what ratio to crop your image with.