Where possible materials for online delivery should be divided into even sized chunks. The reason behind behind this is actually quite simple.
One of the biggest advantages of online learning is flexibility and if a student knows how long one unit will take they can plan their time accordingly and fit studying into their working day. If material is divided unevenly or into chunks that are too big to manage it removes some of that flexibility.
As an a simple example, consider a student who works through unit 1 on their first evening of the course. It takes them 30 minutes to work through the unit, they may then assume that if they set aside 30 minutes the following night, they could work through unit 2. However, when they get to unit 2 they find it contains three times the amount of material that unit 1 had and therefore 30 minutes doesn’t even get them half way through the second unit. They might now have to change their plans for a later night so that they can work through the remainder of the unit in time.
It is important to clarify that we are not saying that the same amount of time needs to be spent on each topic of your course. Far from it, we all know that some topics take longer to cover then others. We are simply suggesting you divide materials into evenly sized work packages/units, each topic can then have any number of units. For example, topic 1 could have 2 units, while topic 2 could have 6. You are still covering the material in the same depth, you are simply making it more flexible for the students to manage.