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!
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