Source and desired output? Changing the resolution means re-encoding. So the fastest would be to use the fastest encoder. A fast resizer will also help, like say AVISynth's simpleresize().
That's a nice tool, but have you considered Quicktime Pro? To me it's the most straightforward. Download the free Quicktime preview from Apple's site. How you get to unlock pro is entirely up to you
Yeah that's true, but i usually use the basic formats like AVI, MOV etc. What formats are you using Nanaki?