If you want to be 100% sure that the selection on the new layer is perfectly positioned I would do the following.
Create a blank layer
Ctrl+a to select all on the new layer
Click on the selection menu and select "Transform Selection"
Hold down Shift + Alt whilst you drag one of the corners
You now have a selection which is perfectly equally positioned from each edge.
Now stroke as suggested by Hawkeye
EDIT - the selection will only be equally spaced if the original is square - if it is rectangular there will be a difference between the sides and the top & bottom - but it will still be even.
If you want to adjust the distance of 2 opposing sides (IE Top & Bottom together - or left and right together) then hold down alt whilst dragging the side handles. this will ensure that the shape will be centrally placed.