I use the Size calculator when resizing video while keeping the aspect ratio or when creating images with a certain aspect ratio.
It will show legal sizes for mpeg, avc, divx etc.
The Frame/time calculator is used for converting between video timecode and video frames.
Basically, if you have no idea what to use this page for then you don't need it.
If you do find it useful you can save it and use it offline. No external files are needed.