I'm not sure if the shortcut is the same in PS9 (I use CS5), but alt+ctrl+i (or Image > Image size under the menu), and you can choose a certain size to resize to (shrink by percentage, by an exact pixel length etc).
You should also have some resample options like bicubic and such, but to be honest I find the difference negligible, so I've never bothered to change it from the default setting.