A Good Tutorial for image resize, canvas(grey and white square screen) and boundary size(image boundary).
"Image>Canvas Size..." [to clip a part]
Right-click on the layer now, and select "Layer to Image Size." The layer boundary (the black and yellow line) should now be the same as the canvas.
Select the Scale Tool (or press Shift+T), and shrink the image to 256x256.
Go to "Image>Fit Canvas to Layers" now the canvas should match with the layer boundary.
