#2- Spending the time thinking about what you want to use it for is probably the most important. I use 3 different notebooks. 1 is my master calendar and weekly to do, 2nd is my notes and 3rd is my business notebook. I started off using a A5 but realized that was just a bit to big for me. My perfect size is a standard. I love my Midori inserts and I can make my own if I would like.
POCKET SIZE- Pocket is commonly referred to as Field Notes size. Field Notes is a popular notebook brand. Many many many friends use a pocket as their life planner and love the portability side of it. I use mine as a wallet and it is perfect!
PERSONAL SIZE- 2nd most popular size. Personal size gives you the portability similar to a pocket size but allows you a little more room for planning. This is a popular size for friends who want to take their planner everywhere with them and use it as a wallet.
NARROW/MIDORI SIZE- Midori is a popular brand and they are the grandfather to the whole Traveler's Notebook empire. If Personal size is just a little to small this may be a great option for you. Inserts are easily available on amazon or etsy and the size will limit you on how many inserts you can use without overhang.
STANDARD LEATHER QUILL/WIDE- Sized to fit Midori, Moleskine Cahier and Maybooks this is our #1 most popular. Inserts are easy to find and make yourself, plenty of room to expand and try new things. It also allows you to keep many inserts in one place.
A5- Essentially this size is a half sheet of paper. Very popular with watercolor artisans, art journal friends and folks who love bullet journaling. Every Artisan does size their products differently but our Leather Quills are perfect for Leuchtturm and Rhodia peeps! Ink Well Press too!!
If you are on the fence about a size buy a insert first! Try the insert and see if it is something you can see yourself using.