I`m working on a website now that provides quality, unique golf lessons via download of video files. The goal was to provide the best quality image with the lowest file size... which means there are trade-offs. Instead of a 1024x768 px size image/playback we use 720x448 which is much larger than the "standard" 300x200 or smaller images you find on a lot of sites.
But the customer is downloading the file to play on their computer...not viewing it as part of the website. The larger size is designed for that purpose.
What size for showing a video on a webpage? It depends. Depends upon if it is a "teaser" clip or the viewer has selected the video to watch and how is fits with the design / purpose of the webpage.
Always, always give the viewer the option to view or not to view the video and stream the video playback.
Well a good standard for video is to make sure it always fits in all available screen resolutions like youtube screen size which happens to be 450x370. Web videos are NOT TV, they are usually lower quality and smaller, which is accepted today. Now if you want a high resolution image you might want to try the divx format great quality but not everyone can use it because they’ll have to download the plug in. For Flash video players there are tons of pre-built ones out there. Like this one "agriya" it does the streaming and everything, it`s very easy to use. For movie conversion you can try this product "Alive Video to Flash converter" Personally I wouldn’t cater to several different bandwidths for movies maybe a low/high.
As for webpage size, a majority of surfers using 1024 however there has been a trend to reduce copy and simplify sites which means sites have reverted back to 800x600 to make it easier on the user. Again, I personally prefer to lock the design down because of the variables involved, i.e., browsers, OS, screen resolutions, yadda yadda yadda. Building flexible sites is cool and if you have the time to build them, great, but for cost effective solutions, I would recommend picking a size and sticking with that. Just imagine building something that always has the horizontal variable of X. Sound fun? Just imagine all the testing you’ll get to do. Not to mention if you use CSS for layout (which you should) it doesn’t handle scaling as well as say tables (yikes did I just say that!)
Now comes Flash the holy grail that ultra web usability junkie’s hate. Aka Jacob Neilson. This format is technically great, because it can reform to any size and scale beautifully from a pda to a 1600x1050 res. You might get some stretching but much easier than your standard webpage. I love flash but only when it’s used properly, so make sure you’re using it when it’s appropriate. Ultimately there is no technical size for video however companies like youtube have set forth a standard that people are used to and sometimes it’s easier to go with the flow.