To crop a paper to new dimensions, to go from 12x12 to something like 4x6, first you want to decide if you want to resize the paper (to preserve more of the pattern) or crop the design as is.
To shrink the whole paper to save as much of the pattern as you can, first go to Image -> Image Size in the menu. In this window you can shrink the whole canvas to the largest of your desired dimensions. So if you want to end up with 4x6, shrink right now to 6x6, by changing the width and height each to 6 inches.
Next you want to go to Image -> Canvas Size in the menu. This will allow you to change the actual dimensions of the paper. Change either the height or width to your desired size.
If you want to maintain the size of the pattern, don't do the first step of shrinking the paper, and skip to the second step of just changing the dimensions with Canvas Size.
Check out this video to watch me resize a digital paper.:
Thank you
Thank you!