image_proc resize is relatively new http://wiki.ros.org/image_proc (the inner page link isn't working) doesn't have a lot of detail but the source cfg file shows all the parameters:
https://github.com/ros-perception/ima...
You need to run a nodelet for it for every destination resolution you want, the nodelet doesn't have to subscribe or publish to other nodelets but there is a performance enhancement if you go to that extra effort.
It looks like it can do either scale factor (e.g. 0.5 for half as big, 5.0 for five times as large) or a destination resolution in pixels.
(TODO I have some aspect ratio preserving code in my own version that inserts black vertical or horizontal letterbox bars that I ought to merge in).