Digital images are defined by pixel count and aspect ratio...
inches don't really mean much except when you are using
a printing template or cropping.....then the dialog may show
4x6, 5x7, 8x10, etc. but not in Paint.
Windows 7 - Crop A Photo Using Paint
http://windows.microsoft.com/en-US/windows7/Crop-a-picture-using-Paint
If you use Windows Live Photo Gallery to crop your photo...
you will have Proportion Choices of 4x6, 5x7, 8x10, etc.
How to Crop Photos by Using Windows Live Photo Gallery