Podsize 0.2 - Popular Pick

According to the official iPod tech specs, video formats for iPod are constrained by maximum video dimensions/resolution of 640x480 for H.264 and MPEG-4 formats. In practice this restriction is actually set at 307,200 pixels (being 640*480) regardless of the video dimensions, the upshot being so long as you stay below this limit - you can encode video in widescreen formats (16:9 etc) at higher resolutions that you might at first think.

Podsize is a widget for Mac OS X 10.4 Tiger that allows you to check the dimensions of your video prior to encoding to ensure it will be compatible with your iPod.
Using Podsize

On the front, enter your chosen video dimensions into the appropriate boxes, select a constraint (4:3, 16:9 etc) and Podsize will let you know whether or not your video is iPod-compatible.

You can also “bump” your video dimensions up and down using the controls at the bottom of the widget.

On the back of the widget, you can swap between H.264 and MPEG-4 video and customise Podsize to match your iPod.