Back in my web developer days I developed a couple of web tools to help determine width's and height's for video inside content columns on websites. Feel free to use them.
16:9 Aspect Ratio Calculator
This tiny web application lets you enter a width of a video in pixels, and it will automatically resize to the right 16 by 9 aspect ratio listing the height as well.
4:3 Aspect Ratio Calculator
For all those super old school creators, here is an aspect ratio calculator for the 4:3 format. It works the same as the 16:9 aspect ratio calculator, just for 4 by 3.