To mark a selection, hold down the left mouse button and move the mouse to size the rectangle. Afterwards, if you would like to move the selection rectangle you can point with the mouse somewhere inside the selected area, hold down the left mouse button and move. If you would like to resize the selection, you can grab any of the eight handles and move the selected edge. The four handles at the corners enables you to move two edges at the same time. The four handles at the center of the edges lets you move only the selected edge.
If you need to make a very precise selection you can zoom into the image with any of the Zoom functions.
Use the Selection Info window to view the coordinates and size of the selected area.
You can cancel the selection by pressing Esc.