After triggering screen capture with ⌘⇧4 (Cmd+Shift+4), you can use additional keys like in Photoshop. Holding down Shift (⇧) will keep lock one dimension, for changes to only the vertical or horizontal dimension. Holding down Option (⌥) will cause the overlay to be sized symmetrically.

Tested only OS X Snow Leopard. Not sure about earlier versions.