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I think you already know the answer because you have explained it correctly when you stated your question. If you are able to select your main object, then go to Select>Modify>Expand to increase the size of that selection. The "expand" command is stated in pixels, so it may be trial & error to estimate how many pixels equals 1/4 inch. (If that calculation needs to be precise, there are ways to do it.) But once you have expanded your selection by the proper amount, create a new layer and go to Edit>Stroke. Apply a black (or whatever color) stroke to your expanded selection and then you're done.

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