What size monthly planner should I buy?

8.5" x 11" is the most common large format and gives you room to write multiple items per day, which is what most verified-purchase reviewers favor for work use. Compact 7" x 9" or smaller sizes are better for portability and bag use. Desk-pad and 9" x 11" large planners are best if the planner lives on a desk rather than traveling with you.