David Allen is very clear in "Getting Things Done" that the only things that should go on the calendar are "hard landscape" appointments that must be done at a specific time, and tasks that must be done sometime that day. Many people using productivity systems based on his vary from that practice. Including me, at least a little.
In my world, all tasks go on a list. The size of the task doesn't have any effect on whether it goes on list or calendar, although if it is a big enough task, I might decide I need to break it down further to get to a real Next Action.
Appointments and "to be done sometime today" are entered on the calendar.
Where I vary somewhat from strict GTD is in blocking time on the calendar for my highest priorities. During weekly review, I'll block out a couple of hours in each of the next couple weeks for the things that are most important for now. That might be a specific deliverable at work, or a project at home that will take multiple work sessions to get done, or simply a note that I need to spend some additional time on a 20kft area of focus. I don't come anywhere close to filling the calendar, but I do make sure there is time reserved for things that might otherwise get skipped on my lists in favor of something else in the heat of doing things. This technique works most effectively when I make entries at a 20kft horizon of focus, rather than a specific project, although I do both. When that blocked time comes, I work on whatever tasks are appropriate for the purpose of the time block, then move on.