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This is not very "automatic", but here's a convoluted way to arrive at what you're trying to do. If you activate the layer with your green shape and select Edit>Transform>Scale, a sizing rectangle is formed around the shape and there is data displayed showing the exact pixel dimensions of that rectangle (see red arrows in the image below).  Once you know these exact pixel dimensions, you can then use the rectangular shape tool on a new layer to create a rectangle then re-size it (Edit>Transform>Scale) to those exact pixel dimensions. Once you've got that, you can create a selection from that layer which will exactly fit your irregular green shape.

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