Now that you've had a chance to think about whether you need a monthly, weekly or daily planner, the next step is to decide what size planner you need. It's not as easy as it sounds.
What size planner you choose is a balance between portability and page space.
If your planner will stay at home or work all the time, it can be as big as you want.
But if you will carry your book everywhere with you, make sure it's a size you can stand to carry in your bag or hands all the time. Everyone has their own size threshold. Do you need a book you can slip into your pocket? Do you carry a big bag and don't mind a large book?
When looking for a planner to take with you everywhere, it's tempting to get the smallest planner possible. But you must make sure the spaces are large enough to write in everything you need. I've had many a Planner Fail due to too-small day spaces.
Something else to consider is the size of your handwriting. If you have tiny, neat handwriting you'll be able to get away with a smaller page size (and therefore smaller book) than someone like me who has large handwriting.
Book size may affect what format of planner you choose. If you need to write lots of details every day and you want to take your book everywhere with you, you may prefer a pocket size day per page planner instead of a desk size weekly planner.
Another size issue has to do with whether you choose a bound book or a ring binder to use as your planner. Ring binders are bulkier and heavier than a bound book of equivalent page size, yet they have some advantages over bound books. Click here for my Ring Binder vs. Bound Book post to help you decide.
What size planner do you use? Do you carry it with you, or leave it at home or work most of the time?