YouTube Video Formats
One great thing about sites like YouTube is their ability to accept many different file formats, making it much easier for home users to participate. You can try uploading almost any format — there's a good chance that it will work. YouTube simply converts it to the FLV format.
However, it does help to understand a few basics and create videos in a format which will work best for your content.
YouTube recommends MPEG4 (Divx, Xvid) with MP3 audio. This option probably provides the best compromise between quality and file size. Other versions of MPEG are also acceptable.
This is the default format used by many editing programs. It provides excellent picture quality but file sizes are large. This can be a good option for short videos.
MOV, WMV & Others
These are perfectly acceptable, but may not provide the same quality or small file sizes as MPEG4.
In most cases you will need to compress your video before you upload it (this may be done automatically by your webcam or editing software). The best approach is to compress files the least amount possible. YouTube will re-compress the file when it is converted to FLV, so you don't need to worry about the file size. If you have a fast connection and plenty of bandwidth, it makes sense to create video files with the least compression while still keeping to the 100MB size limit.
Video Size (Resolution, Pixel Dimensions)
YouTube recommends a resolution of 320x240. Note that this is smaller than the standard YouTube video screen — videos this size are automatically expanded to fill the screen. A quick test indicates that the screen used for the main YouTube player is 450x338 pixels (do not take this as an official specification though).
On the whole, 320x240 is a fairly good compromise which suits the different player sizes well enough. Remember that the video will not always be seen on the large version of the player, and some versions of the player are actually smaller than 320x240.