Choosing your export format
Your choice will depend mainly on 3 factors:
If your audience requires high-quality video - for example because you are showing a procedure where it is essential to see fine detail, then you will need relatively high resolution (probably 640x480 minimum) and relatively little compression. If quality is less of an issue, for example for a talking head (especially if shown with slides), then you might export your video as 320x240 with a lot more compression.
If you are distributing for example on DVD, then you can aim for quite high quality, as a DVD will hold 4.7Gb. A DVD can store a full length high quality MPEG-2 movie at 720x576 resolution, achieving a bitrate of over 4 million bits per second (Mbps).
However if you are delivering over a network, including the internet, then you will need a much lower bit rate, typically for broadband at 340 or 512 thousand bits per second (Kbps).
This will however depend on how long your audience is willing to wait for the video to download. To achieve these lower bitrates you will need to limit the picture resolution to 320x240 pixels, and apply more compression. If you plan to deliver to a handheld device or mobile phone then your bitrate and resolution will of course need to be a lot lower, typically 208x160 pixels.
This will include for example what kind of internet connection your audience has, whether they will have a DVD player available etc