Resizor :: Documentation
Coming Soon…
The documentation is still in the works. But when it's finished, you'll find it here.
But since it does not seem to be as obvious as I had hoped: You need to press the Process-Button (the one with the lightning on it) to actually perform the resize operation.
The software is pretty self-explanatory, but if you want to know more about the difference between resizing and retargeting, you could have a look at the FAQ.