How do I crop the contents of a smart object?

Try this:

1. Select all layers,

2. Merge them to a single layer at top of stack (ctl-alt-shift-e),

3. Crop as needed. I used the selection tool, then (image-crop),

4. Hide smart object below (eye icon) to preserve it's integrity for future use, including applied filters.

This appears to hide the extra space the smart object would otherwise occupy.

I can think of work arounds, but is there an important reason why you need to do this?

I did wonder if making guides at 0% and 100% would show when the SO was opened in a new window, but that didn't work.  What you could do, is

1. Open the SO in a new window.
2. Select all the layers and duplicate back to the original document.
3. You could then initiate a crop, tap on a corner handle to make the crop active and OK it
4. Select the duplicated layers and make them a new SO

Or you could just live with it. The biggest problem with SOs and cropping IME is alignment.  Same thing when you need to edit the SO and need to add canvas to make room for the edit. When you save the edited SO the location has broken.  You can get round this with the same duplicate layers trick, and align them to the original SO, maybe setting the blend mode to Difference (which makes the pixek turn black when perfectly aligned).

Just to confirm @JJMack statement: Smart Object layer can not be cropped. Content of SO layer is in the background and what you see on layer are pixels rendered from original file then usually scaled down. If you want to crop content then you must acess content of SO and that is what @jane-e is suggesting.

Hi @J Isner 

Open the SO by double-clicking the icon in the Layers panel to open the .psb > make edits > save and close the psb to return to the psd.

~ Jane