If you make a rectangular selection in Photoshop, and you find it’s not big enough you COULD make it bigger by using Select > Modify > Expand. This will work if you only need to expand by about 4 or 5 pixels, any more than that and you’ll notice that the corners start to become round.
So here’s a better way to expand;
1. Make your selection.
2. Press Ctrl+T (win) or Cmd + T (mac) to bring up the Free Transform bounding box.
3. Simply click and drag one of the bounding box handles and pull your selection to the required size. Press Enter or Return to complete the transformation and voila! Nice crisp corners on your expanded selection.
You can transform any selection of any shape in this way. I’d love to hear your selection tips, any you’d like to share?