You can also resize a selection rectangle proportionately.
If you need to resize in the other dimension, release the Shift key and then press it again to reset the axis lock. This lets you resize your selection in a single dimension, horizontally or vertically, without changing the other dimension. Press the screen-selection shortcut and select an area of the screen, but then hold down the Shift key.
Mac OS X also offers some options for controlled resizing.
