Do You have a link to the tutorial?
Will your video be displayed anywhere else (e.g. YouTube) other than on one of your proprietary web pages? If not then what works best for You should be the guiding light... along with the following suggestion which is based on H.264 codec (.mp4 format) with which I am most familiar. This codec, of course, is a significant component of the web's video lexicon.
I'm not sure why the 747 x 420. "747" number is weird from a compression efficiency point of view. H.264 does best with dimensions that can be divided evenly by "16" (though "8" & "4" also are good).
A matrix of 16 x 16 pixels is at the core of the H.264 compression algorithm - what is called a macroblock. So in this regard your video dimensions 640 x 480px (= "VGA" resolution) are great. So is 720 x 480px, etc.
Dimension with ratios that deviate from this lose efficiency in compression & quality. But ultimately almost anything works.