A5 & Half-Letter
These have a slightly different page proportion, but they can share the same hole or disc punching. A really useful size for a desktop planner.
You may have a Louis Vuitton GM or a Mulberry Planner in A5, although Mulberry use different hole spacing. You may also have a Franklin Covey Classic planner in half-letter size that uses its own 7 ring punching system.
B6, PW & FCC
These sizes all use the same hole spacing, but even though they are only vary a little in size, they have very different advantages and disadvantages. B6 offers the largest writing area but is the most susceptible to brand interpretations of page dimensions. FCC inserts may work in personal planner covers but are a little harder to shop for. PW is a good next size up from personal size, and may be a good compromise between the three.
Personal & A6
These are very different sizes that use different hole spacing, yet many places mis-label A6 as personal size, leading to many mistakes with ordering. Personal planners are very common and easy to buy for, but A6 planners, sometimes referred to as 'True A6' planners are a little harder to obtain. Personal planners are easily recognised in photos due to their narrow and long page proportion.
You may have a Mulberry Agenda in A6 that uses the standard hole punching, or a Louis Vuitton MM that follows the standard personal sizing.
Pocket Sizes
The standard pocket size is the most common and easy to purchase for, but pocket plus offers a little more room.
You may have a Louis Vuitton PM planner in this size, or a Mulberry Pocket Book which uses a slightly narrower page size. Moterm also label this as A7.
All three pocket sizes use the same hole spacing.
Mini Planning
Filofax Mini planners are the most popular in this size, although you may also know this as a Moterm A8. They use the same hole punch spacing as pocket planners, but they have 5 rings rather than 6. Be careful; older Van der Spek planners in this size have a different 5-ring hole layout.
A9 planners are 3-ring planners that require careful consideration of how to make the space work. Individual punches are normally used for this page size, but 3-hole punches are available. Products for this size are coming soon.