Here's my solution to resizing to a particular pixel size. Open the photo then chose crop tool. Go to tool bar and change mode from unrestrained to custom ratio. I wanted a 640x960 for my iTouch so typed in a radio of 64x96. (On drop down menu click on add preset if you want to use that ratio regularly). Adjust grid to the photo to capture the bit that you want. Go to File>Export and chose Jpeg, and type in the size of jpeg that you want, in my case 640 - 960 populates itself because of the aspect ratio. At this point you can also choose a size larger than the original photo without changing the canvas size. Click on Export and you have your photo at the exact pixel size you want.
