I want my image to be half the size, so I'm going to put 50 for the Width and Height (be sure Percent is selected).
Leave the Print Size where it is.
Check the box next to Resample using: and select Smart Size from the drop down list.
Uncheck the box next to Maintain original print size:
Check the box next to Lock aspect ratio: but do not ajust the number. It will be different for every image depending on size.
Check the box next to Resize all layers.
