I found a great article over at the Editblog as a part of their Useful Tools for Editors series. It's a dashboard widget for OS X that lets you calculate frame sizes for graphics to match a wide range of tape and film formats. The widget is called called Aspect and it's totally free. Grab the widget from this file hosting site by clicking the Download link at the top.
Be sure to stop by and visit the Editblog for lots of news for video pros. The blog is frequently updated and a source of excellent information.