I agree with the other answers: Don't worry too much about the size, try it and improve - that's the Kanban way :-).
To still give you some concrete values, here's what our team does:
Minimum size is about 30 minutes - smaller cards cause too much overhead and clutter the board. If something needs to be done that takes <30 minutes, it (usually) gets added to a related card.
We aim for a maximum of one work day, because ideally you don't want to carry an open task to the next day, but that does not always work out. We sometimes have tasks that take two or three days, maybe even a week, but these are outliers, and we actively try to avoid these.
Finally, if you find that a task is larger than anticipated, it is perfectly acceptable to split it even after you started it. Break of the part that is done, change the card accordingly and finish it, and create a new card for what is left. That way you can already have a partial result (e.g. if you must ship), and the remaining work is visible.