Ideally you would want to crop to the same ratio as your chosen paper. That way you don't have to worry about the physical size as it will happily scale to fit the paper, regardless of the actual physical dimensions.
Work out your ratio and then use the marquee tool (M) in Photoshop. At the top of the screen, under the menu bar, you should see a 'Style' option. Select 'Fixed Ratio' from the drop-down and then input the appropriate values.
Now use the marquee tool to select the area of the photography you want to use. Once selected, click on the 'Image' menu at the top of the screen and select 'Crop'.
That'll give you an image able to scale to any size of paper you so choose. And if that's any of the 'A' paper sizes, your ratio would be 1:1.4142.