...On what your goals for using Kanban are.
What are you trying to accomplish by using a Kanban board?
Are you just needing a way to visualize your work?
Then it doesn't matter.
Are you hoping to improve your flow by limiting Work in Progress (WiP)?
Then keeping them consistent is needed or else your WiP will fluctuate. It doesn't matter if you have a WiP of 5 if what '5' means can vary wildly. The actual size won't much matter, but it's possible to make a larger task smaller by breaking it up but not vice versa, so aim for the lower end of your typical tasks.
Some other reason?
Then think it over, discuss with the team/stakeholders, and come to a decision based on your actual needs.
Many tasks I definitely can split into smaller ones. But what about the task "Be a mentor for a junior developer"?
Kanban is not designed for ongoing tasks. Things like 'fix any bugs that come up', 'ensure everything is always secure' and, in this case, 'answer any questions Alice comes up with' should not be tracked on the board.
However, things like 'Set up Alice's dev environment', 'teach Alice how we use Git', and 'go through our onboarding documentation with Alice' are fine tasks.