My favorite module length is 5-10 minutes total (5 lessons at between 1 and 2 minutes a lesson). Ultimately ending with an activity or two that allow your students a chance to go deeper and apply what they have learned.
If you do it right your 5-10 minute module can become a productive day of practical learning. And if that module is not relevant for your student, they only lose 5-10 minutes of their day.
There is a natural tendency to teach too much. Trying to be too comprehensive. It might work out okay in person, but on the web a little bit of overwhelm will send your student away.