Not having seen an example of how your images look, my theory is:
1) You will have to duplicate the moon layer and add a levels adjustment to get rid of anything other than the moon in the background, then use the magic wand tool without anti-aliasing to select the background, select inverse and crop. Then remove the extra (adjusted copy of original image) layer used for the selection.
Now you should have an image which is cropped tightly around the moon on each side.
2) You can resize the now-cropped image to any specific pixel dimensions. Turn resampling on in the resize image dialog and select pixels as the units.