Your videos can be any length of time from 1 minute up to 65 minutes. However, in order for the class to have an automatic thumbnail photo, the class must be 5 minutes long.
We always suggest posting several different lengths to have variety and see what does best. For fitness, we find that 20-30 minutes is a good length, but our users also love midday workouts around 7 minutes!
If your classes are more educational in nature, you may want to upload multiple shorter classes. You can always group them together by using the same title for each and number them in order so that it appears as a series!