Cropping an image to a layer.

[mwenz]

...Imagine I have

1. a PSD with a document size of 500x500 pixels.
2. It has one layer, upon which sits a green square 50 pixels on each size and sitting at (250, 300).
3. What I want from this document is a PNG, a 50 pixel green square.

I know this is simplistic -it's just an example.

Yeah, the problem with "simplistic" is that (almost) any example I can think of won't likely fit what it is you are really needing.

Screen shot should be self-explanatory. But, just in case, all I did (other than try to recreate your example) was to use the magic wand, inverse the selection, copy to the clipboard and then hit ctrl+n to create a new document. Depending upon the preferences setting, you can see in the screen shot the new document is being made at the clipboard content size, which is your 50 px X 50 px. Then paste it into the new document.

See? A too simple answer for a simplistic question!